This is a very rich sauce that makes a delicious accompaniment to herbed couscous dishes, as well as, grilled vegetables, fish and all meats. Leftovers freeze well. Recipe is taken from my Sauces, Salsas and Marinades cookbook.
Heat a large heavy-bottomed non-reactive pan on the stove and dry-fry the cumin and coriander seeds with the cinnamon stick. Once slightly browned and aromatic, take the spices off the heat and crush in a mortar and pestle or in a spice grinder.
Heat the oil in the same pan and add the onion and garlic. Fry for a few minutes until softened. Add the ground toasted spices and the sumac and cook for a further minute before adding the tomatoes. Half-fill the tomato can with water and add this with all the other ingredients, seasoning cautiously bearing in mind that the sauce will reduce. Simmer, lidded, over a very low heat for 20 minutes. Allow to cool slightly before whizzing up in the blender. Adjust seasoning. Serve hot or cold. Keeps for a week in the refrigerator.