Preheat your oven to 375°F (190°C).

In a large sauté pan, heat the olive oil over medium heat. Add the sliced potatoes and cook for about 10 minutes or until golden and tender.

Add the chopped onion and minced garlic. Sauté for about 3-4 minutes until the onion is translucent.

In a separate bowl, whisk together the eggs, milk, salt, and pepper until smooth.

Spread the cooked potato mixture evenly in the sauté pan and pour the egg mixture over it.

Top the frittata with chopped spinach and grated parmesan cheese.

Cook on the stovetop for an additional 4-5 minutes, until the edges begin to set.

Transfer the pan to the preheated oven and bake for 15-20 minutes until the center is set and the top is golden.

Allow to cool for a few minutes before slicing and serving.