Place the tomato paste in a 2-cup liquid measuring cup or glass jar. Add the salt, garlic powder, mustard powder (if using), cinnamon, and about 5 twists of black pepper. Use a small whisk to stir the ingredients together until they’re evenly combined.
Add the vinegar, and whisk until combined. Add the water, and whisk again until you have a smooth, even mixture. Season to taste with additional pepper, if desired. If you’d prefer a thinner ketchup, add more water by the teaspoon until you reach your desired consistency.
For the best flavor, cover and refrigerate the mixture for at least 1 hour before serving. This homemade ketchup will keep in an airtight jar in the refrigerator for up to 2 weeks.
