Prepare the Egg Mixture

In a large mixing bowl whisk together the eggs, milk and Parmesan cheese until well blended. Add salt and freshly ground pepper, and stir gently to mix. Reserving the mixture for later.

Cook the Onion and Garlic

Heat a non-stick, oven-safe skillet over medium heat and pour in the olive oil. Add the minced onion and sauté for 4 to 5 minutes, or until translucent. Stir through the minced garlic and (fresh chopped) sun-dried tomatoes (if using) and fry for a further minute to release the flavors.

Wilt the Spinach

Working in small batches, add the spinach to the skillet, letting each handful wilt before adding more. Toss the spinach with the onion mixture using tongs until combined. Repeat until all the spinach is added and completely wilted.

Combine with the Egg Mixture

Pour the spinach and onion mixture over the bottom of the skillet in an even layer. Add the egg mixture to the vegetables in an even layer. Carefully lift the edges of the spinach mixture with a spatula so the egg mixture flows under it.

Add Goat Cheese and Cook

Scatter the crumbled goat cheese on top of the frittata. Turn heat down to low, cover the skillet, and cook for 10 to 13 minutes, until the edges are set and the center jiggles slightly.

Broil to Finish

Heat the broiler, and set an oven rack in the top third of the oven. Wearing oven mitts, and taking care not to disturb the skillet too much, transfer the frittata to the oven and broil for 3 to 4 minutes, until the top is lightly golden. When it has puffed up, use oven mitts to remove the skillet from the oven, and let the frittata cool for a few minutes before serving.