Preheat the oven to 375°F (190°C).

In a mixing bowl, whisk together the eggs, milk, salt, and pepper until well combined.

In a skillet over medium heat, sauté the onions and bell peppers until softened, about 5 minutes.

Add the zucchini and spinach, cooking until the spinach is wilted, about 2-3 minutes.

Pour the egg mixture over the sautéed vegetables and cook for 2-3 minutes without stirring, until the edges begin to set.

Sprinkle the cheddar and feta cheese evenly over the top.

Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set and lightly golden on top.

Remove from the oven and let cool for a few minutes before slicing.

Garnish with fresh parsley and basil before serving.