Preheat your oven to 375°F (190°C).

In a large sauté pan, heat a drizzle of olive oil over medium heat. Add sliced medium potatoes and sauté until golden brown and tender, about 10 minutes.

Add chopped medium onion and minced garlic to the pan. Sauté for 3–4 minutes until onions become translucent and fragrant.

Whisk together large eggs, milk, salt, and pepper in a mixing bowl until smooth and well combined.

Spread the cooked potato mixture evenly across the bottom of the sauté pan. Pour the egg mixture over the potatoes.

Sprinkle fresh spinach and grated Parmesan cheese over the top of the egg and potato mixture.

Cook for 4–5 minutes over medium heat until edges start to set while the center remains slightly runny.

Transfer the sauté pan to your preheated oven and bake for 15–20 minutes until fully set and lightly golden on top.

Allow the frittata to cool for a few minutes before slicing.