Heat the olive oil over medium high heat and brown the beef on all sides, working in batches if necessary. Lower heat to medium and stir in the onions and garlic. Add the cinnamon, allspice, sumac, pepper, salt and za'atar. Stir well and cook for a couple of minutes, enjoying the heady fragrance of the spices.
Stir in the tomato pieces and turn heat to low. Cover the pot and cook for 10 minutes or so until the tomatoes have rendered their juices. Add the chopped parsley and distribute the lemon slices around the pot. Cover and simmer for up to 3 hours over very low heat, stirring occasionally and taking care to be gentle with the lemon slices.
When serving, garnish with additional parsley and one of the cooked lemon slices.