Prepare the Potatoes:

1. Cook the Potatoes: Heat 1 tablespoon of olive oil in a large, oven-safe skillet over medium heat. Add the diced potatoes and cook, stirring occasionally, for about 8-10 minutes, or until they’re golden and tender. Remove the potatoes from the skillet and set them aside.

Prepare the Vegetables:

2. Sauté the Onion and Garlic: In the same skillet, add the remaining tablespoon of olive oil. Add the chopped onion and cook for 2-3 minutes, or until softened. Add the minced garlic and cook for another 30 seconds, until fragrant.
3. Add the Spinach: Add the chopped spinach to the skillet and cook for 2-3 minutes, stirring occasionally, until wilted. Season with salt and pepper to taste.

Make the Egg Mixture:

4. Whisk the Eggs: In a medium bowl, whisk together the eggs, milk, and a pinch of salt and pepper. If you’re using Parmesan cheese, whisk it into the egg mixture as well.

Assemble the Frittata:

5. Combine Everything: Add the cooked potatoes back into the skillet with the spinach and onion mixture. Spread them evenly in the pan.
6. Pour the Egg Mixture: Pour the egg mixture over the vegetables, making sure it’s evenly distributed. Let it cook undisturbed for 3-4 minutes, until the edges start to set.

Cook the Frittata:

7. Finish Cooking the Frittata: Transfer the skillet to the preheated oven (at 375°F/190°C) and bake for 12-15 minutes, or until the eggs are fully set and the top is slightly golden. You can also cook the frittata on the stovetop over low heat with a lid on for 10-12 minutes if you prefer not to use the oven.
8. Cool Slightly: Remove the frittata from the oven and let it cool for a couple of minutes before slicing.

Serve and Garnish:

9. Serve and Enjoy: Slice the frittata into wedges and serve warm. Optionally, garnish with fresh herbs like parsley or basil for an added touch of freshness.