Baked Potatoes with Lebanese-Style Beef Filling

3 russet (baking) potatoes (about 1/2 pound each), scrubbed, patted dry, and rubbed with vegetable oil
2 teaspoons cornstarch
2 1/4 cups plain yogurt
1 large garlic clove, minced and mashed to a paste with 1 1/2 teaspoons salt
1/3 cup coarsely chopped walnuts
3 Tablespoons vegetable oil
1 onion, halved lengthwise and sliced thin crosswise
3/4 pound ground chuck
3/4 teaspoon cinnamon
1/2 cup minced fresh parsley leaves

Preheat oven to 450 degrees F. Prick the potatoes a few times with a fork and bake them on a rack in the middle of the oven for 1 hour. While the potatoes are baking, in a small bowl dissolve the cornstarch in 2 Tablespoon of the yogurt. In a saucepan simmer the cornstarch mixture, adding the remaining yogurt, whisking for 5 minutes. Stir in the garlic paste, simmer the sauce, whisking, for 2 minutes. Cover and keep warm. In a skillet cook the walnuts in the oil over moderate heat, stirring, for 1 to 2 minutes, or until they are golden. Transfer them with a slotted spoon to a large bowl. In the oil remaining in the skillet cook the onion, stirring occasionally, until it is golden, add the chuck and brown the meat, breaking up the lumps.

When the potatoes are done, leave the oven on, still set to 450 degrees F. While the potatoes are still warm, halve them lengthwise, scoop out the flesh, leaving crisp shells, and coarsely chop the flesh. In the large bowl combine the walnuts, chuck mixture, chopped potato, cinnamon, parsley and salt and pepper to taste. Divide the mixture among the potato shells and top it with some of the sauce. Bake the potatoes on a baking sheet for 10-15 minutes, or until the filling is hot. Serve them with the remaining sauce. Serves 6 as an entrée.