Preheat oven to 350°. Slice tomato and mozzarella.
Cut open English muffins. Spread the bottom half with a tablespoon of marinara sauce; then top with a slice of roast beef, a thin slice of tomato, a slice of mozzarella, and finally the muffin top.
Place muffins onto a baking sheet and bake for 5 minutes until cheese has melted.