In a large mixing bowl, whisk together the eggs and milk until well combined. Season with salt and pepper to taste.
Heat the olive oil in a large ovenproof skillet over medium heat. Add the chopped onion and bell pepper, and sauté for about 5 minutes until softened.
Stir in the diced zucchini and cook for another 3 minutes. Then add the chopped spinach and cherry tomatoes, cooking until the spinach is wilted.
Pour the egg mixture over the sautéed vegetables in the skillet, making sure it evenly covers the veggies.
Sprinkle the crumbled feta cheese and chopped parsley on top of the mixture.
Allow the frittata to cook on the stovetop for about 3-4 minutes until the edges start to set.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is fully set and lightly golden on top.
Remove from the oven and let it cool for a couple of minutes. Slice into wedges and serve warm.
Notes
Serve each frittata wedge on a plate garnished with more fresh parsley and a dollop of Greek yogurt or a sprinkle of additional feta for an appealing touch.