Preheat your oven to 350°F (175°C) and set the oven rack to the center position. This ensures your frittata bakes evenly later on.

Heat a frying pan over medium heat and add 2 tablespoons of the extra-virgin olive oil. Add the fresh baby spinach and 1/4 cup of water, then cover the pan. Allow the spinach to wilt fully, which should take about 2-3 minutes. Remove the lid, season the spinach with salt and pepper to taste, and transfer it to a mesh strainer. Press the spinach to remove as much excess liquid as possible, then transfer it to a cutting board, chop it coarsely, and set aside.

Heat a 10-inch cast iron skillet over medium heat and add the remaining 4 tablespoons of extra-virgin olive oil. Add the peeled and diced Yukon Gold potatoes and sauté for 10 minutes, stirring occasionally. Then add the diced onion and continue cooking for 3-5 more minutes, or until the potatoes are nearly tender. Season the mixture with salt and pepper to taste. Make sure not to let the potatoes brown too much—just let them become soft. I find it's important to season at this stage so the vegetables carry more flavor into the frittata.

In a mixing bowl, beat the eggs with the whole milk until well combined. Stir in the grated Pecorino Romano cheese, chopped sun-dried tomatoes, and the chopped spinach from Step 2. Mix everything together thoroughly, ensuring that the mixture is uniform and the ingredients are evenly dispersed.

Pour the egg mixture from Step 4 evenly over the cooked potatoes and onions in the skillet (from Step 3), then turn off the stove. Sprinkle the mozzarella cubes evenly over the top. Transfer the skillet to the preheated oven and bake for 20-22 minutes, or until the eggs are set. For even more delicious flavor, I like to let the cheese get slightly golden and bubbly on top.

Remove the frittata from the oven and allow it to settle for a few minutes before slicing. Slice into wedges and serve warm. Enjoy!