Sauté the Vegetables. Preheat a pan to medium-high heat. Slice the onions and pepper thinly. Add 2 tbsps. of olive oil and sauté the onions for 15 minutes, stirring every 3 minutes. If the onions brown too much, add half a cup of water and let them cook. Chop the mushrooms and add them to the onions, cooking for about 5 minutes. Season with salt and pepper, then set aside.
Toast the Bread. Cut the bread in half and toast it in a pan over medium-high heat.
Cook the Redefine Flank Steak. Heat 2 tbsps. oil in a skillet over medium-high heat. Sear the steak in the hot pan for approximately 3 minutes per side until a caramelized crust forms. Slice the steak into 1 cm thick pieces and sear again until thoroughly cooked. Set aside.
Assemble the Sandwich. Spread mayonnaise on both sides of the toasted bread. Place the sliced flank steak on the bread and top with the onion and mushroom mixture. Optionally, add Swiss cheese on top of the onions and mushrooms. Cut the sandwich in half and serve.