Melt butter with oil in a large heavy-bottomed pan with a tight-fitting lid.
Sweat onions and garlic gently over low heat for 15 minutes.
Add potatoes and stir to coat in the onion and garlic mixture.
Season generously with salt and freshly ground black pepper.
Cook potatoes and onions for 15 minutes on low heat, adding a little stock if they stick.
Heat a frying pan and render the reserved lamb fat with seasoning.
Remove solid fat pieces and brown lamb chops on both sides in the frying pan.
Transfer the browned chops into the pan with the potatoes and onions.
Deglaze the frying pan with a little stock and add the liquid to the stew.
Pour in remaining stock to just below the ingredients and simmer covered for 90 minutes.
Add chopped parsley 10 minutes before the end of cooking if desired.
Rest the stew for 10 minutes and adjust seasoning before serving.