Place tomatoes and onion in a large stainless steel saucepan and cook over moderate heat for 30 minutes or until the tomatoes are soft. Strain the mixture through a large sieve, stirring as much of the pulp through as possible. Place the strained tomatoes back in the saucepan and whisk in remaining ingredients. let mixture simmer over moderate heat for 1 1/2 to 2 hours, stirring occasionally, until the mixture has thickened to the consistency of ketchup. place in sterilized Mason jars and refrigerate; it will keep for several months.
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