1. Heat the skillet over medium-high heat and add olive oil.
2. Season the steak with salt and black pepper, then cook in the skillet for 3-4 minutes on each side.
3. Remove the steak and let it rest. In the same skillet, add sliced onions and bell peppers. Cook until softened.
4. Slice the steak thinly. Toast the bread slices.
5. Assemble the sandwich by placing the steak slices on one slice of bread, topping with onions and bell peppers, and covering with the other slice of bread.