Preheat your oven to 375°F (190°C). Gather all ingredients and equipment.

In a large skillet over medium heat, add a drizzle of olive oil and sauté finely chopped onions and minced garlic for about 5 minutes.

Add diced zucchini, bell peppers, and tomatoes to the skillet. Cook for another 7–10 minutes until tender.

In a large mixing bowl, combine the sautéed vegetables with shredded cheese, eggs, and fresh spinach.

Transfer the mixture into the greased casserole dish, spreading it evenly.

Bake in the preheated oven for 30–35 minutes until the top is set and golden.

Allow the casserole to cool for about 5 minutes before slicing and serving.