Cut the hearts in half crosswise. Remove the veins, skin and blood, wash the hearts carefully, season with salt and pepper and brush with mustard.
Put the fat in a flat baking dish. Place the hearts in it, cover with slices of bacon and add the onion slices. Put the pan on the wire rack in the oven and cook at 200-220 degrees for approx. 60 minutes. 10 minutes before the end of the roasting time, pour the cream (canned milk) over the hearts.