In the northern parts of Greece, all sorts of meats, from chicken to savory meatballs to lamb and goat, are cooked in sauces based on the local, delicious, thick, strained sheep’s milk yogurt. This dish comes from the traditional cooking of the Vlachs, a once itinerant shepherd tribe, and is sometimes called vlachiko. I have modified this recipe by using boneless lamb instead of leg of lamb and replacing some of the butter with olive oil when browning the meat and vegetables.