Preheat the oven to 350 degrees F.

Beat together the eggs with some salt and pepper in a bowl. Stir in the cheese and set aside.

In a large cast-iron skillet, add olive oil over medium-high heat. Add the potatoes and cook until slightly browned. Then add onion, stirring frequently, until soft and golden brown, several minutes. Add the spinach and cook until wilted. Then add the bacon until warm.

Make sure all the ingredients are evenly distributed across the bottom of the skillet, then pour in the egg mixture so that it evenly coats everything, then put the skillet in the oven. Cook, watching it carefully, until the eggs are set but not very brown on top, 20 to 30 minutes.

Slide the frittata out of the skillet and onto a cutting board. Slice into wedges with a long serrated knife.