In a small saucepan, whisk together the tomato paste, apple cider vinegar, and water over medium heat.
Add the onion powder, garlic powder, ground mustard, salt, and black pepper. Stir well to combine.
Bring the mixture to a simmer and cook for about 5-7 minutes, stirring frequently.
Remove the saucepan from the heat and let the ketchup cool for a few minutes.
Taste the ketchup and add liquid stevia or another sugar substitute to achieve your desired level of sweetness.
Transfer the ketchup to a jar or airtight container and store it in the refrigerator. It will keep for about 1-2 weeks.