Cut the tofu into bite-sized pieces.
Slice the onion and bell pepper.
Heat cooking oil in a pan and sauté the onion and bell pepper.
Add tomato sauce to the sautéed vegetables and bring to a boil.
Layer tofu and sauce alternately in a gratin dish.
Sprinkle mozzarella cheese evenly on top.
Bake in a 200°C oven for 10 minutes.