In a skillet add the butter and beer, heat over medium low heat. Once the butter is melted add the sliced onion and mushroom. Stirring occasionally cook the onions and mushrooms beer and butter have been absorbed and the onions are rich brown. Once done remove from heat. Now it is time to assemble the sandwich

Butter the outside of 2 slices of French bread. Top the inside of one of the slices with the mozzarella cheese, making sure that the whole slice is covered by the cheese. Top the cheese with the onions and mushrooms. Place the slices of tomatoes over the onions and mushrooms. Over the tomatoes top with additional cheese. Place the second slice of bread over the cheese, making sure that the butter side is on the outside.

Heat a skillet over medium to medium low. Once the skillet is heated place the sandwich inside. Cook sandwich slowly so that the cheese melts and the bread doesn't brown too quickly. Once the sandwich is brown on the bottom flip over to brown the other side. At this point it is a good thing to place a cover or something to weight the sandwich down a bit. Continue to cook until the sandwich is golden brown and cheese is melted and gooey. Serve immediately.