Depending on the size, halve or quarter tomatoes so that they’re approximately 1 – 1 1/2″ pieces.
Place in a large stockpot. Using a wooden spoon or a potato masher, gently mash the tomatoes to release their juices. Cook the tomatoes on a low simmer for 30 minutes.
Using an immersion blender or a regular blender, puree the tomatoes until they are mostly smooth.
Pour tomato puree into a slow cooker. Add remaining ingredients and cook on low overnight, or 8-12 hours. Using an immersion blender or a regular blender, puree the mixture again until it is mostly smooth.
Using chopsticks or dull knives, prop open the lid of the slow cooker on opposite sides so that the lid does not touch the slow cooker. Cook the mixture on medium/high setting until the mixture has cooked down to the desired thickness, checking every hour or so. This takes approximately 3 hours.
Taste ketchup and add additional seasoning or sweetness as desired. (I ended up adding one more tablespoon of honey.)
Using an immersion blender or a regular blender, puree the mixture one final time.
Ladle or scoop into glass jars for storage. Alternatively, you can re-use old ketchup bottles for your new, homemade ketchup.