Lunch & Dinner

French Onion Tartine

4 servings
Sandwich roll with combo of mushrooms, onions and cheese for filling. Thyme sprig garnish. Sitting on white sandwich paper and gray wood-grain table.

The flavors of French Onion Soup, but in a shroom-filled sammie!

Truffles are a type of fungi that grows underground. Not a mushroom per se, but the earthy-umami vibe is big and bold in these knobby, odiferous little critters. A little goes a long way, so truffle-infused oil, salt, hot sauce, and other condiments are usually where you can indulge relatively cheaply and easily. Truffle hot sauce with this sammie brings another layer of fun-gi to the party. 

Ingredients

  • 1 lb crimini mushrooms, thinly sliced 
  • 2 cloves garlic, minced 
  • 2 lb sweet onions, halved lengthwise and thinly sliced 
  • 1 bay leaf 
  • ¼ tsp dried thyme 
  • ¼ cup dry white wine 
  • ¼ tsp Worcestershire sauce 
  • ½ tsp Sherry vinegar 
  • 6 oz Gruyère cheese, grated 
  • ¼ cup grated Parmesan 
  • 1 tsp salt, divided 
  • ½ tsp pepper, divided 
  • 3 Tbsp butter 
  • 3 Tbsp olive oil, divided 
  • ½ tsp sugar 
  • ½ cup mayonnaise 
  • 1 Tbsp Dijon mustard 
  • 4 deli sandwich rolls (like Mexican bolillo bread), or wide baguette bread (1 1/3 baguette) 
  • Optional: Truffle hot sauce, fresh chopped parsley – for serving 

Directions

  1. Melt butter in a large sauté pan over medium heat. Add 1 Tbsp olive oil, onions, ½ tsp salt, ¼ tsp pepper, sugar and bay leaf. Cook uncovered, stirring occasionally, until onions are golden brown and caramelized, 45-50 minutes. Turn heat down and/or deglaze pan with a little water as needed so onions don’t stick and burn. Remove from pan and set aside. Clean out pan. 
  2. In the large sauté pan, add 2 Tbsp olive oil. Once it’s warmed up, add garlic and cook 1 minute. Add mushrooms, dried thyme, Worcestershire sauce, ½ tsp salt, ¼ tsp pepper. Toss to coat and stir occasionally. Once mushrooms are pretty well cooked, deglaze pan with white wine. Remove from pan. 
  3. Mix together mayonnaise, Dijon and sherry vinegar.  
  4. Split open rolls, or baguette bread portioned into sandwich size, Toast in oven under broiler until lightly golden. 
  5. Assemble sandwiches: Spread mayo mix on bread. Top with caramelized onions and then mushrooms. Top each sandwich with grated cheeses and return to broiler until cheese melts. Add optional truffle hot sauce and chopped parsley if using. Serve.