Preheat your oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat, then add the chopped onions.
Add the diced potatoes to the skillet and season with salt and pepper.
Cook the potatoes and onions together for about 10-15 minutes until the potatoes are tender, stirring occasionally.
Crisp the bacon in the skillet, then crumble it and set it aside.
In a bowl, whisk together the eggs, milk, and a pinch of salt and pepper until well combined.
Add the cooked onion and potato mixture to the egg mixture, followed by the crumbled bacon and cheddar cheese.
Pour the mixture back into the skillet and cook for a few minutes until the edges set.
Transfer the skillet to the preheated oven and bake for 20-25 minutes or until the frittata is set and golden.
Remove from the oven and let it cool slightly before slicing and serving.