Warm a skillet over medium heat for 5 minutes. Add steak strips and cook for 3 minutes on each side, or until outsides are browned. Remove from heat and set aside. Cover to keep warm. Add the pepper strips and onion slices to the skillet and cook for 4 minutes, or until just soft. Place pepper-onion mixture in a bowl and cover to keep warm. Keep the skillet on the burner over medium heat.
Place the bread slices on a flat work surface. Spread 1 tablespoon of butter evenly over one side of each slice of bread.
Place two of the bread slices onto the hot skillet, butter side down. Top one slice with 1/2 cup shredded Swiss cheese (or 2 slices) and the other with 1/2 cup shredded provolone cheese (or 2 slices). Cook for about 3 to 4 minutes, or until the cheese begins to melt around the edges.
Using a spatula, move the bread from the skillet back to the work surface. Top the Swiss cheese with 1/2 of the steak, 1/2 of the pepper-onion mixture and the other cheesy bread slice, cheese side down. Repeat this process with remaining ingredients. Cut in half and serve!