Remove the stems from the mushrooms and clean them thoroughly.

Finely chop the onion, yellow bell pepper, orange bell pepper, and the removed mushroom stems.

Heat a pan over medium-high heat and sauté the chopped vegetables.

Add tomato spaghetti sauce to the sautéed vegetables and cook until the mixture is thick and moisture-free.

Fill the cleaned mushroom caps with the cooked vegetable mixture.

Sprinkle mozzarella cheese generously over the filled mushrooms.

Bake in a preheated oven at 200°C (392°F) for 8-10 minutes until the cheese melts and turns golden brown.