In a large heavy pot heat the olive oil over moderate heat and cook
the onion until golden brown, stirring occasionally, about 8 to 10
minutes. Add the garlic and cook for another minute. Add the tomatoes,
vinegar, corn syrup, cloves, allspice, mustard seeds, celery seeds,
salt and pepper stirring to combine. Bring the mixture to a boil,
reduce heat and simmer, stirring occasionally, for 1 hour. Pass the
mixture through a food mill into a bowl and return to the pot. Bring
the mixture back to a simmer and cook until very thick, stirring
occasionally, about 45 minutes. Allow the ketchup to cool. The ketchup
can be sealed in sterilized jars and kept indefinitely.
