2 (28 ounce) cans crushed tomatoes
½ cup water, divided
⅔ cup white sugar
¾ cup distilled white vinegar
1 teaspoon onion powder
½ teaspoon garlic powder
1 ¾ teaspoons salt
⅛ teaspoon celery salt
⅛ teaspoon mustard powder
¼ teaspoon finely ground black pepper
1/8 cayenne pepper
1 whole clove
Pour tomatoes into slow cooker. Swirl 1/4 cup water in each emptied can and pour into the slow cooker. Add sugar, vinegar, onion powder, garlic powder, salt, celery salt, mustard powder, black pepper, cayenne pepper, and whole clove; whisk to combine.
Cook on High, uncovered, until mixture is reduced by half and very thick, 10 to 12 hours. Stir every hour or so.
Use an immersion blender for about 20 seconds to smooth the texture of the ketchup.
Ladle ketchup into a fine strainer and press mixture with the back of a ladle to strain out any skins and seeds.
Transfer strained ketchup to a bowl. Cool completely.
Taste and adjust salt, black pepper, and cayenne pepper as desired.