Canning method
Wash and rinse pint canning jars; keep hot until ready to use. Prepare lids according to the manufacturer’s directions.
Place the pickling spice on a clean, double-layered 6-inch square of 100% cheesecloth. Bring the corners together and tie with clean string, or use a purchased muslin spice bag.
Wash and peel tomatoes by placing washed tomatoes in boiling water for 1 minute, immediately transferring them to cold water, and slipping off the skins. Chop into 1/2-inch pieces. Peel, wash, and dice onions into 1/4-inch pieces. Wash, core, and seed bell peppers; chop into 1/4-inch pieces. Combine the tomatoes, onions, and peppers in an 8- or 10-quart Dutch oven or saucepot.
Wash, peel, and pit peaches; halve and soak for 10 minutes in an ascorbic acid solution made with 1,500 mg ascorbic acid in 1/2 gallon water. Wash, peel, and core apples; halve and soak for 10 minutes in the same type of solution. Quickly chop peaches and apples into 1/2-inch cubes and add them to the saucepot.
Add the pickling-spice bag. Stir in the salt, red pepper flakes, brown sugar, and vinegar. Bring to a boil while stirring gently. Reduce heat and simmer for 30 minutes, stirring occasionally. Remove and discard the spice bag.
Using a slotted spoon, fill salsa solids into hot clean pint jars, leaving 1 1/4-inch headspace. Cover with cooking liquid, leaving 1/2-inch final headspace.
Remove air bubbles and adjust headspace if needed. Wipe jar rims with a damp clean paper towel and apply two-piece metal canning lids.
Process pints in a boiling-water canner for 15 minutes at 0–1,000 feet, 20 minutes at 1,001–6,000 feet, or 25 minutes above 6,000 feet. Cool undisturbed for 12 to 24 hours and check seals.