Rinse the soft tofu with water and set aside.
Finely chop the onion and bell pepper.
Heat a pan, add olive oil, and sauté the onion and bell pepper.
Season with salt and pepper, then add tomato sauce and let it simmer.
Place the soft tofu in a gratin dish and top with the sautéed sauce.
Evenly sprinkle mozzarella cheese on top.
Bake in a preheated oven at 200°C (392°F) for 10 minutes until done.