01 - Preheat oven to 375°F.
02 - Heat olive oil in a 10-inch ovenproof skillet over medium heat. Sauté the diced onion for 3 to 4 minutes until translucent. Add minced garlic and cook for 1 additional minute until fragrant.
03 - Stir in the chopped spinach and sauté until just wilted, approximately 2 minutes. Add the sliced roasted red peppers, stir to combine evenly, and remove the skillet from heat.
04 - In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, dried oregano, and red pepper flakes if using. Fold in the crumbled feta and half of the shredded mozzarella or Gruyère.
05 - Pour the egg mixture over the sautéed vegetables in the skillet. Gently stir to distribute the fillings evenly throughout. Sprinkle the remaining shredded cheese over the top.
06 - Transfer the skillet to the preheated oven. Bake for 20 to 25 minutes until the center is fully set and the top is lightly golden brown.
07 - Remove from the oven and let the frittata rest for 5 minutes before slicing into wedges. Serve warm or at room temperature.