Begin by washing and chopping the zucchini, bell peppers, onion, and garlic into small, uniform pieces.

In a large skillet, heat a tablespoon of olive oil over medium heat. Once hot, sauté the chopped onion and garlic for about 2-3 minutes.

Add the bell peppers and zucchini to the skillet and cook for an additional 5-7 minutes, stirring occasionally.

In a large mixing bowl, combine the sautéed vegetables with the tomato sauce, spinach, and half of the cheese. Stir thoroughly.

Preheat your oven to 375°F (190°C). Spread the veggie mixture evenly inside a greased baking dish and top with the remaining cheese.

Bake for 30-40 minutes, or until the cheese has turned golden brown and the edges are bubbling.

Remove from the oven and let it cool for about 10 minutes before serving.