Preheat the broiler. Toss the onion slices with the olive oil
them until charred, about 8 minutes on each side.
Put the onions
and all the remaining ingredients in a deep heavy nonreactive pot. Simmer
uncovered over low heat for about 3 hours, stirring every 15 minutes to break up the tomatoes and to keep the ketchup from sticking to the bottom of the pot. The mixture will thicken.
Puree the ketchup in batches in a food processor
. If the puree seems too thin, return it to the pot and simmer until it is as thick as you like it. This ketchup keeps well refrigerated for up to 4 weeks.