Add all of the ingredients to a small saucepan. Simmer over low heat, stirring occasionally with a wooden spoon to prevent burning, for 10 minutes or until thick and reduced by about 1/3.
Remove and discard the onion. For a smoother sauce, you can blend the ketchup with an immersion blender or food mill, but it isn't necessary.
Scrape the ketchup into an airtight container and let cool at room temperature, then refrigerate for a least an hour, until chilled.
Notes
I love the extra layer of flavor that celery salt adds to this recipe. If you don't have it, that's ok. Leave it out and add an extra pinch of Kosher salt.