Step 1: 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 black pepper, stirring gently.
• Set this mixture aside.

Step 2: Cook the Onion and Garlic

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

Step 3: Wilt the Spinach

• Add 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.

Step 4: 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.

Step 5: 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.

Step 6: 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.