Cut the tofu into bite-sized pieces.
Finely chop the onion and bell pepper.
Heat olive oil in a pan and sauté the onion and bell pepper.
Add the tofu to the pan and lightly stir-fry.
Season with salt and pepper.
Place the sautéed ingredients in a gratin dish and pour tomato sauce over them.
Generously top with mozzarella cheese.
Bake in a preheated oven at 180°C for 15 minutes.