Clean the leeks and discard the rough green parts. Mince the liver and leeks or onions together, preferably in a food processor. Put mixture in a bowl and stir in the suet, breadcrumbs, sage, mint and seasoning to taste.
Form the mixture into 12 balls with floured hands and place in a well-greased, shallow ovenproof dish. Pour the stock into the dish, cover and bake for about 30 minutes.
Uncover and continue cooking for a further 10 minutes or so to brown the meatballs.
The remaining gravy may then be thickened, if desired. Serve with mashed potatoes, peas or green beans.