Steps
1. Heat olive oil in a non-stick skillet over medium heat.
2. Add the sliced onions and cook slowly for about 10 minutes, stirring occasionally until they are golden and caramelized.
3. In a bowl, whisk together eggs, milk, salt, pepper, and chopped parsley.
4. Pour the egg mixture over the caramelized onions in the skillet.
5. Cook over low heat until the edges set, about 4 minutes.
6. Sprinkle Parmesan cheese on top if using.
7. Transfer skillet to a preheated oven at 375°F (190°C) and bake for 5-7 minutes until the frittata is fully set and lightly golden on top.
8. Remove from oven, let cool slightly, then serve warm.