Put the chicken stomachs in a saucepan, cover with lightly salted water or vegetable stock, bring to the boil and simmer for about 40 - 60 minutes. If you like the stomachs softer, you can let them cook longer.
Then drain, catch the broth and strain it, you will still need it. Cut the stomachs into bite-sized pieces.
Heat the butter and oil in a pan, fry the stomachs and onions in it and deglaze with cognac. Add 150 ml of stock, season to taste and stew for about 25 minutes. Add the cream just before the end of the cooking time. Possibly thicken with a little sauce thickener.
If you like a lot of sauce, you can of course add more stock and cream. The cognac can also be left out or replaced with broth.
Rice tastes good with it, but also pasta or mashed potatoes.