In a mixing bowl, add eggs, milk, and parmesan cheese. Mix them well and season with some salt and pepper. Keep it aside.
In an oven-proof skillet, add some olive oil and place it over medium heat.
Add the onions and cook them until they turn translucent for about 4-5 minutes.
Add some garlic and sun-dried tomatoes and cook it for about a minute more.
Add some chopped spinach and cook them along with onions; as the spinach wilts, add more.
After the spinach has reduced in volume, add this egg mix with parmesan over its top.
With the help of a spatula, make sure the mixture reaches the bottom of the pan.
Also, add the small pieces of goat cheese over the frittata mixture and cover the pan.
Let it cook for 10-13 minutes until you see that it is almost cooked and has taken a proper shape. If the center of the frittata is set, it is cooked.
Preheat your broiler and keep the rack in the top third row of the oven.
Cook it for about 3-4 minutes until the top of the frittata turns golden in color. Take it out and let it cool a little bit. Then, serve it.