To make the sweet potato purée: Preheat the oven to 400°F. Line a baking dish with aluminum foil. Prick each sweet potato a few times with a fork, and bake until very tender, about 1 hour. When cool enough to handle, peel the sweet potatoes, break them into large chunks, and purée in a food processor with the yogurt, salt, and vanilla until smooth. (The sweet potato purée will keep for a few days in the refrigerator; rewarm before serving.)

To make the meatballs: Combine the American lamb, garlic, salt, paprika, cumin, cinnamon, and pepper in a large bowl and mix to thoroughly combine. Roll 2-tablespoon-size balls of the mixture to yield about 24 meatballs.

Heat the oil in a large frying pan over medium heat. When the oil is shimmering hot, add the meatballs in a single layer with room between them. (Cook in two batches if needed.) Let them cook without disturbing until a crust forms on the bottom side, about 2 minutes, then continue cooking, stirring often, until nicely browned on all sides and cooked through, 10 to 12 minutes.

To assemble: Toast the flatbreads in a 375˚F oven until lightly browned around the edges and crisp, 3 to 5 minutes. Top each flatbread with a smear of the sweet potato purée, about 6 meatballs, a handful of arugula leaves, and the optional toppings of your choice. Serve hot or at room temperature with apple cider.