Cut the tofu into bite-sized pieces.
Slice the onion and bell pepper thinly.
Heat oil in a pan and sauté the onion and bell pepper.
Add tomato sauce and simmer lightly.
Place the tofu in a gratin dish and pour the sauce over it.
Top with mozzarella cheese.
Bake in the oven at 200°C for 10 to 15 minutes.