In a bowl, combine the meat with 2 teaspoons of cumin and the parsley, salt, pepper and cinnamon. Squeeze any excess liquid from the grated onion; add to the meat. Mix well. Form the meat into eight 1/2-inch-thick oval patties.
Melt the butter in a small skillet. Add the garlic and cook over moderate heat until fragrant but not browned, about 1 minute. Transfer to a small bowl and whisk in the yogurt. Season with salt and pepper. In the same skillet, warm the tomato sauce over moderate heat and stir in the cayenne and the remaining 2 teaspoons of cumin.
In a nonstick skillet, heat the oil until shimmering. Add the kefta and cook over moderate heat, turning occasionally, until cooked through, 10 minutes.
Place the pitas on 4 plates and top with the kefta. Spoon some tomato and yogurt sauce over the kefta and serve, passing the remaining sauce at the table.