In northern 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, at one time an itinerant shepherd tribe, and is sometimes called vlachiko.
Preheat the oven to 350 degrees.
Heat the olive oil and butter in a large, wide pan over medium-high heat and brown the lamb pieces, turning to color them evenly. Add the scallions and cook, stirring, until soft. Stir in the garlic and cook for a few minutes to soften. Add the rice and pour in the wine and water. Cover, bring to a boil, and reduce to a simmer. As soon as the rice has absorbed most of the liquid, season the contents of the pot with salt and pepper, to taste, and remove from the heat.
Whisk the eggs in a medium-sized bowl until foamy. In a separate bowl, whisk together the yogurt and flour. Whisk the eggs into the yogurt mixture and stir in the chopped mint. Season lightly with a little salt.
Spread out the lamb mixture in a casserole dish with a lid or divide evenly between individual ramekins, about 3 inches deep. Spread the yogurt mixture evenly over the meat. Bake, uncovered, until the yogurt is set and lightly browned on top, for 45 minutes. Cover and cook until the lamb is tender, for 45 more minutes. (If using ramekins, check after 25 minutes.) Ten minutes before removing from the oven, sprinkle, if using, with grated kefalotyri cheese. Serve immediately.