Rinse the soft tofu in water and drain well.
Finely chop the onion and bell pepper.
Heat olive oil in a pan and sauté the chopped onion and bell pepper.
Add tomato sauce and season with salt and pepper.
Place the soft tofu in a gratin dish and top with the sautéed sauce mixture.
Sprinkle mozzarella cheese evenly over the top.
Bake in a preheated oven at 200°C (392°F) for 10 minutes.