Heat a nonstick or cast-iron skillet over medium heat. Add the chopped bacon and cook until it becomes crispy and releases its fat. Remove the bacon and place it on paper towels to drain, but keep about a tablespoon of bacon grease in the pan for extra flavor.

Add olive oil to the skillet with the bacon fat. Toss in the diced potatoes and cook for about 8–10 minutes, stirring occasionally, until they’re golden brown and tender.

Add the chopped onion and cook for another 2–3 minutes, until the onion softens and becomes fragrant.

In a medium bowl, whisk together the eggs, milk, salt, and pepper. Stir in the shredded cheese and cooked bacon (save a little for topping).

Pour the egg mixture evenly over the potatoes and onions in the skillet. Tilt the pan slightly to make sure the eggs cover everything. Cook over low heat for 5–7 minutes, until the edges start to set but the center is still slightly runny.

Transfer the skillet to a preheated oven at 375°F (190°C) and bake for about 10 minutes, or until the top is puffed and golden. If you like a crispier top, you can broil it for 1–2 extra minutes at the end.

Let the frittata rest for a few minutes before slicing. Sprinkle with chopped parsley or green onions for a fresh touch, and serve warm.