Heat the oil in a large frying pan, add the onions and potatoes, then the liver and the bacon. Season as you go. Cover with a tight-fitting lid and cook on a low heat for 20–30 minutes until the potatoes are just tender.
Meanwhile mix the mustard with salt and pepper to taste (and a little sugar if you like), add the vinegar and oil and amalgamate. Mix the avocado with the dressing. Put