Heat the oil in a 3-5 quart saucepan and add the onion. Cook over medium-low heat, stirring occasionally until soft and golden, about 10 minutes. Add the remaining ingredients, and stir to combine. Bring to a boil, lower heat, and simmer, stirring occasionally for 15 minutes, until thick. Blend the ketchup in batches in a blender or use an immersion blender until smooth. Transfer the ketchup to a bowl and allow to cool completely before tasting to adjust seasoning.