Step 1: Preheat your oven to 180°C (350°F).

Step 2: Heat olive oil or butter in an oven-safe skillet over medium heat. Add chopped onions and cook for 3–4 minutes until soft. Add garlic and optional vegetables or meats and sauté for another 3–4 minutes.

Step 3: In a mixing bowl, whisk together eggs, milk or cream, Parmesan, Pecorino Romano (if using), chopped parsley, basil, thyme, salt, and black pepper until well combined.

Step 4: Pour the egg mixture evenly over the sautéed vegetables in the skillet. Cook over medium-low heat for 3–5 minutes until the edges start to set.

Step 5: Transfer the skillet to the preheated oven and bake for 10–15 minutes, or until the frittata is puffed, golden, and fully set in the center.

Step 6: Remove from the oven and allow the frittata to rest for 2–3 minutes. Slice and serve hot or warm.