Heat a cast iron skillet on medium heat. Add the bacon or pancetta and cook until browned. Remove the bacon, leaving the grease in the pan and drain on a paper towel. Add the sliced, cleaned leeks (make sure they are dry) to the bacon grease and saute until caramelized, about 6-8 minutes. At this point you can add more butter/olive oil if needed – I added a tiny bit of bacon grease I had stored in the fridge.
Add beans and bacon back into the skillet. Stir for about a minute. At this point you can either fry some eggs in a separate skillet, or push the hash to the sides and fry the eggs in the middle. Make sure the beans, leeks and bacon are thoroughly mixed.