Thickening your ketchup: The ketchup will thicken as it cools and sets in the fridge. But if you'd like it to be extra thick, after step five, transfer the ketchup back to its saucepan. Bring it to a low simmer. Mix 2 TBSP of cornstarch into 1/4 cup of cold water, then slowly whisk the mixture into the ketchup until you reach the consistency you're after.