In a large mixing bowl, whisk together the eggs and milk until well combined. Season with garlic powder, salt, and pepper.
Heat olive oil in a non-stick, oven-safe skillet over medium heat. Add the red onion and bell pepper, sautéing for about 2-3 minutes until softened.
Add the zucchini and cherry tomatoes to the skillet and cook for another 3-4 minutes, stirring frequently. Finally, stir in the chopped spinach and cook until wilted, about 1-2 minutes.
Pour the egg mixture over the sautéed vegetables, ensuring everything is evenly distributed in the skillet.
If using, sprinkle the crumbled feta cheese evenly on top of the eggs and vegetables.
Cook on the stovetop for about 3-5 minutes until the edges start to set.
Transfer the skillet to the preheated oven and bake for about 15-20 minutes, or until the center is set and lightly golden.
Remove from the oven and let it cool for a few minutes before slicing.
Garnish with fresh herbs before serving.
Notes
Feel free to customize the vegetables based on your preference.