Juice your tomatoes, garlic, and onion. Pour juice into a large crockpot.
Add vinegar, sugar, salt and spices. Stir to mix. A whisk works well for this.
Turn crockpot to high and leave ketchup to cook, uncovered, until it reduces by half. Stir occasionally.
When ketchup has reduced, add cornstarch and stir to mix.
Continue to cook until the ketchup reaches your desired texture.
Eat right away, freeze, or process half-pint jars in a boiling water bath for 20 minutes.
