Combine red bell peppers, onions, sugar, vinegar, and salt in a large pot; bring to a boil. Reduce heat and simmer until vegetables are tender, about 30 minutes. Sterilize the jars and lids in boiling water for at least 5 minutes. Pack relish into hot, sterilized jars, filling to within 1/2 inch of the top.
Grind or otherwise process peppers and onions into finely chopped, even mushy, consistency. Cover with boiling water and let stand 10 minutes. Drain well. Save juice to add to vinegar mixture if needed to cover vegetables. Add sugar, vinegar, celery seed and salt. Boil 15 minutes.
Prepare all three varieties of relish to give as a thoughtful gift set to the host. For a quick and easy keto tartar sauce recipe: Mix equal portions of mayonnaise and Garlic Lover’s Relish (recipe below). Season to taste with sea salt and freshly ground pepper.
In a large nonreactive kettle, combine the two kinds of vinegar, sugar, mustard seeds, and paprika, if using. Add the well-drained pepper mixture and bring to a boil. Reduce heat to medium-low and simmer, uncovered, for 50 to 60 minutes, until thickened. Stir occasionally.