Preheat your oven to 375°F (190°C). Have your oven-safe skillet ready.

In a large mixing bowl, whisk together the eggs, milk or half-and-half, grated cheese, salt, pepper, and optional red pepper flakes until well combined and slightly frothy.

Heat 1 tablespoon of olive oil in the oven-safe skillet over medium-high heat. Add the diced potatoes and cook, stirring occasionally, for 8-10 minutes, or until tender and lightly browned. Remove the potatoes from the skillet and set aside.

Add the remaining 1 tablespoon of olive oil to the same skillet (add a little more if needed). Reduce heat to medium. Add the chopped onion and cook for 3-5 minutes until softened and translucent.

Add the minced garlic and cook for another minute until fragrant, being careful not to burn it. Add the fresh spinach in batches if necessary, stirring until it completely wilts, about 2-3 minutes.

Return the cooked potatoes to the skillet with the onion and spinach, distributing them evenly over the bottom of the pan.

Pour the egg mixture evenly over the vegetables in the skillet.

Let the frittata cook on the stovetop for 2-3 minutes until the edges start to set and firm up.

Carefully transfer the skillet to the preheated oven. Bake for 18-22 minutes, or until the frittata is puffed, golden brown on top, and a knife inserted into the center comes out clean.

Remove the frittata from the oven and let it rest for 5 minutes before slicing and serving. This allows it to set fully.