Cut the tofu into bite-sized pieces.
Chop the onion and bell pepper.
Heat olive oil in a pan and sauté the onion and bell pepper.
Add the tofu and stir-fry together.
Pour in the tomato sauce and simmer slightly.
Transfer the sautéed mixture to a gratin dish and top with cheese.
Bake in the oven or air fryer until the cheese melts.