Prepare the Egg Mixture:
In a large mixing bowl, whisk together the eggs, milk, and Parmesan cheese until thoroughly combined. Add the salt and freshly ground pepper, stirring gently. Set this mixture aside.

Cook the Onion and Garlic:
Heat olive oil in an oven-safe non-stick skillet over medium heat. Sauté the chopped onion for 4-5 minutes until translucent. Add the minced garlic and sun-dried tomatoes (if using) and cook for an additional minute.

Wilt the Spinach:
Introduce the spinach to the skillet in batches, adding handfuls at a time. Use tongs to toss the leaves with the onion mixture. As the spinach wilts and reduces in volume, add more until all the spinach is incorporated.

Combine with the Egg Mixture:
Spread the spinach mixture evenly across the skillet. Pour the prepared egg mixture over the vegetables, ensuring it is evenly distributed. Use a spatula to gently lift the edges of the spinach mixture, allowing the egg mixture to seep beneath.

Add Goat Cheese and Cook:
Scatter the crumbled goat cheese evenly across the top of the frittata. Reduce the heat to low, cover the skillet, and cook for 10-13 minutes, or until the edges are set, but the center remains slightly jiggly.

Broil to Finish:
Preheat the broiler and position the oven rack in the upper third of the oven. Transfer the skillet to the oven and broil for 3-4 minutes, or until the top becomes lightly golden. Remove the skillet from the oven and let the frittata cool for several minutes before serving.