Place the halved potatoes in a large pan, cover with cold water and salt. Bring to boil, simmer for 15 minutes until tender.
Meanwhile, over a low heat warm the oil in a frying pan. Add the leeks and onion fry for 15 minutes until soft. Add the bacon and fry for another 5 minutes until brown.
Drain the potatoes and mash. Stir in the butter and milk. Season well.
Add the egg, stir to combine.
Fold in the leek mixture, paprika and half the cheese.
Add the mixture to a buttered casserole dish.
Sprinkle over the remaining cheese and bake for 45 minutes, until well browned.