Heat the oil in a medium saucepan over medium heat. Add the onion and sauté until just beginning to brown, about 8 minutes, stirring occasionally. Add minced garlic and cook an additional 30 seconds.
Add the tomato puree, brown sugar, vinegar, tomato paste, salt, mustard, cloves, allspice, and cayenne pepper to the saucepan and stir. You need to be stirring it or the tomato sauce will bubble and gurgle and spatter all over your stove. No one wants that! Bring to a boil, then reduce heat to low. Partially cover to prevent aforementioned spattering and simmer for about an hour (or until mixture has thickened to the consistency of ketchup).
Use an immersion blender to puree the ketchup until smooth. Alternately, you can dump the contents of the saucepan into your blender or food processor and puree it that way.
Eat your ketchup!
Notes
Nutrition facts are for the full batch of balsamic ketchup.
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