Created by QueenBea on May 1, 2020
Step 1: Melt 1 tablespoon of the butter in a large skillet, add the onions and mushrooms and cook until onions are soft and translucent; Pour into a bowl and set aside.
Step 2: Butter the outside of each slice of bread.
Step 3: Place two slices of bread, butter side down, in the skillet.
Step 4: Top these two slices of bread with one slice of cheese; cover each with roast beef.
Step 5: Divide the onion mixture evenly on top of roast beef, then add the mustard and mayonnaise.
Step 6: Top with remaining two slices of bread, butter facing out.
Step 7: Cook over medium-high heat, about 2 1/2 to 3 minutes per side, or until golden brown and cheese is melted.
Step 8: Enjoy