01 - Preheat oven to 375°F (190°C).
02 - Heat olive oil in a 10-inch oven-safe skillet over medium heat. Add chopped onion and diced red bell pepper; sauté for 3 minutes until softened.
03 - Add diced zucchini and cook for 2 minutes. Stir in halved cherry tomatoes and cook for an additional 2 minutes. Add baby spinach and sauté until wilted, about 1 minute.
04 - In a mixing bowl, whisk together eggs, whole milk, salt, black pepper, dried oregano, and fresh basil. Stir in half of the crumbled feta and grated Parmesan cheese.
05 - Pour the egg mixture evenly over the sautéed vegetables in the skillet. Cook without stirring for 3 to 4 minutes until the edges begin to set.
06 - Sprinkle the remaining feta and Parmesan over the top. Transfer the skillet to the preheated oven and bake for 10 to 12 minutes until the center is set and lightly golden.
07 - Allow the frittata to cool for 5 minutes before slicing. Serve warm or at room temperature.