Cut the meat into bite-size pieces, trimming off and discarding any excess fat.
Combine the lemon juice, garlic, salt, 1 tablespoon of the sumac and 1 tablespoon of olive oil in a large mixing bowl. Add the lamb and toss to coat evenly. Marinate for 30 minutes at room temperature.
Drain the meat, discarding the marinade, then pat it lightly with paper towels.
Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large, heavy skillet over medium-high heat.
Once the butter has melted and become foamy, add the marinated lamb and stir to coat.
Increase the heat to high; cook about 10 minutes, stirring often, until the meat is well browned on all sides, with crisped edges, and cooked through. Season with 1 tablespoon of the sumac and cinnamon. Taste, and adjust the seasoning as needed. Remove from the heat.
When ready to serve, divide the hummus among individual salad-size plates, using the back of a spoon to spread each portion into a 4-inch round, turning the plate as you go. Create a well at the center of each portion, then fill it with a big spoonful of the lamb. Garnish each serving with parsley, a generous drizzle of oil and a sprinkle of sumac. Serve right away with pita for scooping.