Place the steak in a large ziplock bag or baking dish along with all the marinade ingredient. Toss well to coat. Refrigerate for two hours or overnight.

Remove the flank steak from the fridge and allow it to come to room temperature. In the meantime, place a cast iron skillet on the grill, and preheat it to medium high heat.

Add the butter and 1 Tablespoon olive oil to the skillet. Add the onion and pinch of salt and toss to coat. Put the lid on the grill and let the mixture cook for 15 minutes, stirring about every five minutes. If the onions start to brown before they soften, add a Tablespoon of water to the skillet.

Add the mushrooms and an additional Tablespoon of olive oil to the skillet and cook an additional 5-10 minutes or until onions are soft and sweet and mushrooms are golden brown. Remove the skillet from the grill.

Meanwhile, place the flank steak on the grill over the highest heat source and grill for 4-5 minutes per side for medium steak. Remove to a plate and allow to sit at least 10 minutes before slicing.

Preheat the broiler to low heat. Slice the baguettes in half, smear with butter, and toast them if desired. Layer each sandwich with the steak, the mushrooms and onions, and then the sliced cheese. Place them under the broiler for 3-4 minutes or until cheese is melted. Meanwhile, in a small bowl whisk together the mayo, sour cream, and horseradish.

When the sandwiches are done broiling, spread the top half with mayo mixture and add sliced tomatoes. Sandwich and serve.