Heat 1 tablespoon of extra-virgin olive oil in a 10-inch oven-safe nonstick skillet over medium heat. Add the thin potato slices and sliced onions. Season lightly with salt and pepper. Cook, turning occasionally, for 10–12 minutes until the potatoes are tender and lightly golden.

Clear a small space in the skillet, add the remaining oil if needed, and sauté the minced garlic for 30 seconds. Add the baby spinach and cook for 2 minutes until completely wilted. Remove the spinach and squeeze out all excess moisture using a clean towel.

In a large bowl, whisk the eggs, milk, half of the shredded mozzarella cheese (1/2 cup), Italian seasoning, salt, and pepper until uniform and frothy.

Layer the cooked potatoes and wilted spinach evenly across the bottom of the skillet. Pour the egg mixture over the top, tapping the pan gently so the egg settles into all gaps. Cook undisturbed on the stovetop over medium-low heat for 5 to 6 minutes until the edges are completely set.

Preheat your oven's broiler. Sprinkle the remaining 1/2 cup of mozzarella cheese across the top. Transfer the skillet under the broiler and broil for 3 to 4 minutes until the top is puffed, set, and golden-brown.

Let cool for 5 minutes before slicing into 6 wedges. Garnish with fresh herbs if desired.