Grilled Flank Steak Sandwiches With Mushroom & Onion Dipping Sauce
A bistro-inspired steak sandwich featuring juicy grilled steak, umami mushrooms, sweet caramelized onions, and a savory dipping sauce.
If sandwiches are your comfort food of choice, the Grilled Flank Steak Sandwich is destined to become a favorite. Inspired by French bistro classics and elevated by a homemade mushroom-onion dipping sauce, this recipe pairs juicy steak and earthy mushrooms with crusty baguette for a truly satisfying meal. Whether you’re hosting a dinner party or craving something decadent and comforting, this sandwich transforms a simple cut of beef into an unforgettable treat.
Why This Sandwich Stands Out
- Bistro-Style Experience: The flavors are reminiscent of a classic French steak dinner, complete with a rich mushroom-onion gravy.
- Versatility: Serve it as a sandwich, open-faced with melted cheese, or simply as steak with gravy.
- Easy to Elevate: Pair with homemade pomme frites for a true bistro-at-home vibe.
Ingredients
For the Flank Steak
- 1 pound flank steak
- 3 tablespoons Herbs de Provence
- Salt and freshly ground black pepper
- 2 tablespoons canola oil
- 3 tablespoons olive oil
For the Mushroom & Onion Dipping Sauce
- 1 tablespoon unsalted butter
- 8 ounces baby bella mushrooms, sliced
- 1 medium onion, thinly sliced
- 2 tablespoons olive oil
- 3 sprigs thyme
- 1 dry bay leaf
- 1/4 teaspoon salt
- 1 3/4 cups beef stock
- 1 tablespoon flour
- 1/8 teaspoon pepper
To Serve
- 1/2 baguette, sliced lengthwise
- Optional: Gruyere cheese (if making an open-faced sandwich)
Step-by-Step Directions
Preparing & Grilling the Flank Steak
- Preheat a grill pan over medium-high heat. Lightly coat the pan with canola oil.
- Rub flank steak on both sides with Herbs de Provence, olive oil, and a generous sprinkle of salt and freshly ground black pepper.
- Transfer the steak to the prepared pan and grill for 7 minutes per side for medium-rare, or adjust for preferred doneness.
- Remove steak from grill and let rest in a shallow dish, covered with foil, for at least 10 minutes.
Making the Mushroom & Onion Dipping Sauce
- In a medium skillet, gently melt butter and olive oil over low heat.
- Add sliced onions, bay leaf, thyme sprigs, salt, and pepper. Cook slowly on low heat for at least 20 minutes; if you have more time and patience, 40 minutes yields even sweeter, richer onions.
- Increase heat to medium. Add sliced mushrooms and cook for another 8 minutes, until mushrooms are soft and starting to caramelize.
- Stir in the flour and cook for 1 minute, ensuring no lumps remain.
- Pour in beef stock and whisk to combine. Bring to a boil, then reduce to a simmer for 20 minutes, stirring occasionally.
- Season to taste. Remove thyme sprigs and bay leaf before serving.
Assembling The Sandwich
- Slice the rested flank steak against the grain into thin pieces for maximum tenderness.
- (Optional) Toast the baguette for added crunch and warmth.
- Layer steak slices on the bottom half of the baguette.
- Spoon mushroom-onion gravy generously over the steak.
- Top with the other half of baguette. Slice to serve two large sandwiches.
- Serve additional gravy in ramekins on the side for dipping.
- Bonus: Create an open-faced melt – lay steak on baguette halves, top with Gruyere, broil until cheese is bubbly, then add the mushroom-onion sauce.
Flavor & Technique Spotlight
| Component | Technique | Result |
|---|---|---|
| Flank Steak | Herb Rub, Quick Grilling | Juicy, tender, aromatic steak with subtle floral notes from Herbs de Provence |
| Mushroom & Onion Sauce | Caramelization, Slow Sauté | Sweet, umami-packed gravy with velvety texture |
| Baguette | Toasting (optional) | Crunchy exterior, chewy interior (classic French sandwich base) |
| Open-Face Melt | Broiling | Melty, bubbly Gruyere finish for extra richness |
Tips for the Perfect Steak Sandwich
- Slice Against the Grain: This makes even a tough cut like flank tender enough for a sandwich.
- Caramelize Slowly: Low and slow is key for onions that are sweet and golden, not bitter.
- Sauce Variations: This gravy works equally well as a sandwich dip or spooned straight atop steak and mashed potatoes on non-sandwich days.
- Baguette Quality: Seek out a rustic baguette with good chew for that authentic café feel.
Serving Suggestions
- Make it French Bistro Night at Home: Pair with homemade pomme frites, a leafy salad with vinaigrette, and a glass of red wine.
- Open-Face Variation: Add Gruyere cheese and broil for 1 minute for a croque-style twist.
- Gravy-Only Option: Skip the bread and serve steak with the mushroom-onion sauce, mashed potatoes or rice.
- Customize Your Bread: While baguette is classic, ciabatta, sourdough, or even Texas toast make excellent alternatives.
Nutritional Information
Flank steak is relatively lean yet flavorful, making this sandwich a satisfying but not overly heavy meal. Mushrooms and onions add fiber, vitamins, and antioxidants, while the use of olive oil and moderate butter keeps the fat content balanced. For a lighter option, use whole-grain bread and reduce the sauce quantity.
Frequently Asked Questions (FAQs)
Q: Can I prepare the steak and sauce ahead of time?
A: Yes, you can grill the steak and make the sauce up to a day in advance. Store separately, refrigerate, and gently reheat the sauce before assembling sandwiches.
Q: What other cuts of beef work for this recipe?
A: Skirt steak or hanger steak are excellent alternatives and remain tender when sliced thinly against the grain.
Q: How do I get the most flavor from the gravy?
A: Let the onions cook as long as possible on low heat to deepen their caramelized sweetness. Consider adding a splash of dry sherry or red wine for extra depth.
Q: Is there a vegetarian version of this sandwich?
A: Substitute steak with thick slices of roasted portobello mushrooms or eggplant, and use vegetable stock in the dipping sauce for a hearty vegetarian option.
Q: How should I serve leftovers?
A: Leftover steak and sauce make excellent fillings for wraps, or as a topping for baked potatoes or grain bowls. Reheat gently and add fresh bread or grains.
Expert Tips & Variations
- Herb Riffs: If you don’t have Herbs de Provence, combine dried rosemary, thyme, marjoram, and a hint of lavender if available.
- Texture Add-Ons: Layer in sautéed spinach, roasted bell peppers, or crispy fried onions for additional flavor and texture.
- Dipping Sauce Boost: Stir a teaspoon of grainy mustard into your mushroom-onion gravy for added zing.
- Cheese Lovers: Besides Gruyere, aged cheddar or provolone melt beautifully atop steak and gravy.
Make It a Meal: Pairing Ideas
- Classic French Fries: Serve your sandwich with homemade pommes frites or thick-cut steak fries for a true bistro pairing.
- Vegetable Sides: Roasted green beans, glazed carrots, or an arugula salad balance out the richness.
- Wine Pairings: A medium-bodied red wine, such as Merlot or Cabernet Franc, complements both the beef and the umami mushrooms.
Recipe Summary Table
| Step | Action | Time Required |
|---|---|---|
| Steak Marinade & Grill | Season, grill, rest, slice | ~20 min (plus resting) |
| Mushroom-Onion Sauce | Caramelize, sauté, simmer, finish | 50 min (can overlap with steak) |
| Sandwich Assembly | Toast bread, layer ingredients, serve | 10 min |
Final Thoughts
This Grilled Flank Steak Sandwich with Mushroom & Onion Dipping Sauce is more than just a meal—it’s an experience. Whether enjoyed on a quiet night at home, served open-faced with cheese, or elevated with gourmet sides, each bite delivers rich flavor and comfort. Try this recipe to create your own bistro-style feast.
