Step 1: Preheat your oven to 375°F (190°C). In a large bowl, whisk together the eggs, milk (if using), salt, and pepper until just combined and slightly frothy.
Step 2: Heat the olive oil in a 10-inch oven-safe, non-stick skillet over medium heat. Add the diced onion and cook until softened and translucent, about 3-5 minutes.
Step 3: Add the diced bell pepper to the skillet and cook for another 5-7 minutes, until it begins to soften. Ensure vegetables are cooked through for best texture in your frittata recipe.
Step 4: Stir in the chopped spinach and halved cherry tomatoes. Cook for just 1-2 minutes, until the spinach wilts. Remove the skillet from the heat and spread the vegetables evenly across the bottom.
Step 5: Sprinkle about two-thirds of the crumbled feta (or other cheese) over the cooked vegetables. Pour the whisked egg mixture evenly over the vegetables and cheese.
Step 6: Return the skillet to the stovetop over low heat for 2-3 minutes to allow the bottom of the frittata to begin setting. This prevents sticking and helps create a firm base.
Step 7: Sprinkle the remaining cheese over the top of the frittata. Transfer the skillet to the preheated oven. Bake for 18-25 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. Avoid overbaking.
Step 8: Carefully remove the frittata from the oven. Let it rest in the skillet for 5-10 minutes before slicing and serving. Garnish with fresh parsley or chives, if desired. Serve warm or at room temperature.