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 cook until golden brown.
Place the ingredients in a gratin dish and pour tomato sauce over them.
Top with mozzarella cheese.
Bake in a preheated oven at 180°C (356°F) for 10 minutes until done.