Place the tomatoes, sugar, vinegar, onion, tomato paste, garlic, salt, celery salt, mustard, pepper and paprika into a slow cooker. Cook on high for 8 to 10 hours, stirring occasionally.
Place a couple of layers of cheesecloth in a fine mesh strainer over a medium-sized bowl. Pour the ketchup into the cheesecloth and strain off the solids, such as garlic bits and tomato seeds, which can then be discarded. Press through the strainer with the back of a spoon, if required.
Place the ketchup in a small saucepan and bring to a simmer until thickened to the desired consistency. Add a touch of water if necessary. Season again with additional salt if desired.
Store in a mason jar in the refrigerator for up to 6 weeks. The ketchup can be used immediately after making, but the flavours will improve even more if left overnight.
Nutrients per 1 tbsp serving: 39 calories, 0 g fat (0 g saturated fat), 136 mg sodium, 9 g carbohydrates (1 g fibre, 7 g sugar), 1 g protein