Not everyone's cup of tea - or gravy - but I've recently re-discovered ox liver. Our butcher sells it ready sliced. I buy a lot, enough for 4-5 meals for the 2 of us.
1) make about 2 pints meat stock with 2 stock cubes and some tomato paste, dissolved in boiling water.
2) In a bowl mix 1tbsp flour , s&p. coat the liver slices in this.
3) Heat some oil in a large stovetop to oven pot and fry gently lots of thinly sliced onins, various kinds (white, red, shallots etc.)
4) brown off the liver slices, pushing the onions to one side.
5) Add the stock, scraping the tasty bits from the bottom of pot. Then bake in a med. oven for about an hour.
Not too long or the liver will be dry and grainy.
Let it cool then freeze in separate plastic bags or containers,