1 flank steak
1/4 cup olive oil
3 garlic cloves, minced
2 tablespoons red wine vinegar
1/3 cup soy sauce
2 tablespoons honey
1/2 teaspoon ground pepper
onions and mushrooms:
1 tablespoon butter
2 tablespoons olive oil, divided
1 yellow onion, peeled and thinly sliced
8 ounces sliced mushrooms
pinch of salt
4 ciabatta rolls (or a loaf cut into sections)
8 slices white cheddar cheese
1/4 cup mayonnaise
1/4 cup sour cream
2 tablespoons creamed horseradish spread
Place the steak in a large zip-top bag along with all the marinade ingredients, seal bag and squish to combine and coat. Refrigerate 2 hours or overnight.
Remove steak from fridge and allow to come to room temperature. In the meantime, heat a cast-iron skillet on the grill over medium-high. Add butter and 1 tablespoon of the olive oil. Add the onion and a pinch of salt and toss to coat. Put the lid on the grill and let mixture cook for 15 minutes, stirring about every 5 minutes. Add the mushrooms and an additional tablespoon of olive oil to the skillet and cook an additional 5-10 minutes until onions are soft and mushrooms are golden. Remove skillet from grill. Remove the steak from the marinade and place on the grill in the hottest spot and grill for about 5 minutes each side (depending on the thickness of your cut). Remove from grill and allow to rest for 10 minutes before slicing.
Preheat the broiler. Slice the ciabatta rolls or loaf in half. Layer bottom halves with sliced steak, onions and mushrooms, and then sliced cheese. Place on a rimmed baking sheet and place under the broiler for 3 minutes until cheese melts. Meanwhile, in a small bowl, combine the mayonnaise, sour cream, and horseradish. Spread the top bun with mayo mixture. When sandwiches are done broiling, remove from oven, and top with spinach, tomato slices, and top bun. Slice in half and serve.