Add butter to a large skillet over medium heat. Melt butter and add onions. Cook, stirring occasionally, for about 7 to 10 minutes. Add in the pancetta around the 7 minute mark and continue to cook for a few more minutes.
Whisk eggs, Parmesan cheese, salt and pepper in a large bowl. Pour the egg mixture on top of the onion and pancetta and do not stir. Cook for about 3 minutes on the stove.
Place the skillet in the preheated oven and bake for 15 minutes. Let cool for 5 minutes before cutting into slices.