Wash and chop the tomatoes into quarters.
In a large pot, heat olive oil over medium heat.
Add the chopped onions and sauté until soft, about 5 minutes.
Add the chopped tomatoes to the pot.
Stir in the apple cider vinegar, brown sugar, salt, and spices.
Bring the mixture to a boil, then reduce heat to low.
Simmer uncovered for about 45 minutes, stirring occasionally.
Once thickened, remove from heat and let it cool slightly.
Puree the mixture using an immersion blender or a regular blender.
Strain the ketchup through a fine-mesh sieve for a smoother texture, if desired.