1. Prepare the hearts. Dry thoroughly and fill with the stuffing.
2. Skewer or sew together the thin flaps at the top to hold in the stuffing.
3. Heat the fat in a small baking tin and put in the hearts.
4. Baste well, and bake them in a moderate oven, 180°C, Gas 4, for 1 hour for sheep's hearts, 45 minutes for lamb's hearts.
5. Baste them several times during cooking.
6. When cooked, transfer them to a warmed serving dish, remove the skewers, if necessary, and keep hot under buttered greaseproof paper.
7. Drain most of the fat from the tin but keep any meat juices.
8. Blend a little stock into the flour to make a paste.
9. Mix the paste into the pan juices, then gradually stir in the remaining stock.
10. Heat to simmering point, stirring all the time, and simmer until thickened. Season well.
11. Pour a little of this gravy round the hearts and serve the rest in a warmed sauce boat.