Preheat your oven to 375°F (190°C). If your skillet is not naturally non-stick, lightly grease a 9-10 inch oven-safe skillet with a little olive oil or butter.

Heat 1 tablespoon of olive oil in the oven-safe skillet over medium heat. Add the chopped yellow onion and cook for 3-4 minutes until softened and translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Add the diced bell pepper and zucchini to the skillet. Cook for 5-7 minutes, stirring occasionally, until the vegetables are tender-crisp. Stir in the chopped spinach and cherry tomatoes, cooking just until the spinach wilts, about 1-2 minutes. Remove the skillet from the heat.

In a large mixing bowl, whisk together the large eggs, milk (or half-and-half), salt, and black pepper until well combined and slightly frothy. Stir in half of the crumbled feta cheese.

Evenly spread the cooked vegetables in the bottom of the skillet. Pour the egg mixture over the vegetables. Sprinkle the remaining feta cheese over the top.

Transfer the skillet to the preheated oven. Bake for 20-25 minutes, or until the frittata is set in the center and lightly golden brown on top. A knife inserted into the center should come out clean.

Remove the frittata from the oven and let it rest for 5 minutes before slicing. Garnish with fresh chopped chives, if desired. Serve warm or at room temperature.