1. Slice 1 (8 ounce) package of mushroom into thin pieces. Slice half of a yellow onion as well.
2. Heat a large, deep skillet up over medium high heat. Melt 2 Tb butter in the pan.
3. Add the mushrooms and onions and saute until the onions are translucent and the mushrooms are golden. (Should take about 5-10 minutes).
4. Add 1 Tb soy sauce and 1/4 tsp garlic salt. Give it a nice stir and allow it to cook for 20 more seconds or so. Give the mushrooms a taste and add salt and pepper to your liking.
5. Heat a skillet or griddle up over medium high heat.
6. Slice 4 pieces of french bread (about 1 inch thick). Spread butter over one side of each piece.
7. Take two pieces of bread and place Swiss cheese over the non-buttered side. Add half the mushrooms to each piece, top with provolone cheese and another piece of bread (butter side up).
8. Place the sandwich on the griddle or hot pan. Allow the bottom to become all grilled and golden brown. Then flip the old girl over and let it get grilled on the other side as well.