Rinse the soft tofu under water and drain well.
Finely chop the onion, bell pepper, and mushrooms.
Heat olive oil in a pan and sauté the onion, bell pepper, and mushrooms.
Add tomato sauce and simmer while mixing with the ingredients.
Place the soft tofu into a gratin dish and pour the sauce over it.
Generously top with mozzarella cheese.
Bake in a preheated oven at 180°C (356°F) for 10–15 minutes until the cheese turns golden brown.