Harvest. Pick small, hard green tomatoes from healthy vines before any frost. Pick the peppers.
Wash. Wash the tomatoes and peppers well.
Weigh. Weigh the tomatoes, peppers and onions to match the recipe. Swap sweet peppers only by weight.
Trim. Trim the onions, tomatoes and peppers. Remove pepper stems and seeds.
Grind. Grind the tomatoes, peppers and onions in a food processor in batches, pulsing to a fine relish texture. Transfer to a large pot.
Brine. Dissolve ½ cup canning salt in 1 quart water and pour over the vegetables.
Boil and drain. Bring to a boil and simmer 5 minutes. Drain in a colander. Return the vegetables to the pot.
Cook. Stir in the sugar, cider vinegar, mustard and cornstarch. Bring to a boil and simmer 5 minutes.
Prepare jars. Wash pint jars, set them on the canner rack, cover with water by 1 inch, and boil 10 minutes (0–1,000 ft; add 1 minute per extra 1,000 ft). Keep the jars in the hot water until filling.
Fill. Fill hot pint jars with hot relish, leaving ½-inch headspace. Remove air bubbles.
Wipe rims. Wipe each rim with a clean cloth dampened with white vinegar.
Lids. Center lids with a lid magnet. Screw bands on fingertip tight.
Process. Process in a boiling water canner: pints 5 minutes at 0–1,000 ft; 10 minutes at 1,001–6,000 ft; 15 minutes above 6,000 ft.
Cool and check seals. Cool undisturbed 12–24 hours. Check seals. Refrigerate any unsealed jars.
