Preheat the oven: Set your oven to 400º F (200º C) to get it ready for baking.Prepare the ingredients: slice the vegetables and grate the cheese.Prepare the egg mixture: In a large bowl, whisk together the eggs, Greek yogurt, and a pinch of salt until smooth and even. Set this mixture aside. Sauté the vegetables: Heat the butter in an ovenproof stainless steel or cast iron skillet over medium heat. Once the butter is melted and bubbling, add the sliced baby peppers, mushrooms, and olives. Sauté for about 2 minutes until they start to soften.Add the spinach and tomatoes: Add the fresh spinach and cherry tomatoes to the skillet. Cover the skillet briefly (about 1 minute) to allow the spinach to wilt slightly. Combine with egg mixture: Pour the egg mixture evenly over the sautéed vegetables in the skillet. Gently place chunks of feta cheese on top without pressing them into the eggs. Sprinkle the grated mozzarella cheese evenly over the top.
Bake the frittata: Transfer the skillet to the preheated oven. Bake for 15 to 20 minutes until the edges are set and golden brown. You can check for doneness by gently shaking the skillet—the frittata should be firm and not jiggly in the center.
Finish and serve: Remove the skillet from the oven and let the frittata cool for a few minutes. Sprinkle with red pepper flakes and garnish with fresh herbs of your choice.
Slice into wedges and serve warm or at room temperature. Enjoy!
