Preheat a waffle iron or 400 degree F oven.
Brush one side of each slice of bread lightly with olive oil. With the oil-side down, arrange slices of beef on four slices of bread. Top with mushrooms if desired, then a big dollop of sauce in the middle. Place cheese over the sauce, and then top with another slice of bread with the oiled-side up.
Place sandwiches in a hot waffle iron for a few minutes until heated through. Or, place assembled sandwiches on a baking sheet and bake in preheated oven for 10-15 minutes until hot and toasted. Serve right away.
- Prep Time: 5
- Cook Time: 10