Preheat your oven to 375°F (190°C).
In a mixing bowl, whisk together the eggs, milk, salt, black pepper, and garlic powder until well combined.
Heat the olive oil in the skillet over medium heat. Add the zucchini and bell pepper. Sauté for about 5 minutes until softened.
Add the cherry tomatoes and spinach to the skillet, stirring until the spinach has wilted, about 2 minutes.
Pour the egg mixture over the sautéed vegetables, making sure it is evenly distributed.
Sprinkle the crumbled feta cheese on top.
Cook on the stovetop for about 3-4 minutes, gently lifting the edges of the frittata to allow uncooked egg to flow underneath.
Transfer the skillet to the preheated oven and bake for 20 minutes, or until the frittata is set and slightly golden on top.
Remove from the oven and let it cool for a few minutes before slicing.
Serve warm or at room temperature, garnished with fresh herbs if desired.