Place onion slices on grill. Grill until the slices are blackened on both sides -- about 15 minutes per side.
In small black skillet toast the coriander seeds, cumin seeds, and mustard seeds about 5 minutes, and then grind them in a mortar and pestle or a spice grinder.
In saucepan combine the blackened onions, the toasted spices, red wine vinegar, brown sugar, garlic, capers, and hot sauce.
Add the paprika, cinnamon, allspice, ginger, oregano, black pepper, cardamom, and salt to taste. Then add the whole tomatoes and tomato paste. Simmer this mixture for 3 hours, stirring every 15 minutes to break up the tomatoes and to prevent sticking. Puree some of the mixture in a blender and return to the pot. If the pureed ketchup seems too thin, continue cooking it until it's reduced to a consistency you like.
Recipe courtesy of David Rosengarten