1. Working in 2 batches, pulse cucumbers in food processor until coarsely chopped into ¼-inch pieces, 8 to 10 pulses; transfer to large bowl. Pulse bell pepper until coarsely chopped into ¼-inch pieces, about 6 pulses; transfer to bowl with cucumbers. Pulse onion until coarsely chopped into ¼-inch pieces, about 10 pulses; transfer to bowl with vegetables. Stir in salt, cover, and refrigerate for 3 hours.
2. Set canning rack in large pot, place four 1-cup jars in rack, and add water to cover by 1 inch. Bring to simmer over medium-high heat, then turn off heat and cover to keep hot.
3. Drain vegetables in colander, transfer to dish towel, and squeeze to remove excess liquid. Bring vinegar, sugar, mustard seeds, celery seeds, and turmeric to boil in Dutch oven over medium-high heat. Add vegetables, reduce heat to medium, and simmer until vegetables are translucent and mixture has thickened slightly, 10 to 15 minutes.
4. Place dish towel flat on counter. Using jar lifter, remove jars from pot, draining water back into pot. Place jars upside down on towel and let dry for 1 minute. Using funnel and ladle, portion relish into hot jars, leaving ½ inch headspace. Slide wooden skewer along inside of jar, pressing slightly on relish, to remove air bubbles.
5a. For short-term storage: Let relish cool to room temperature. Cover, refrigerate, and serve. (Relish can be refrigerated for up to 4 months; flavor will deepen over time.)
5b. For long-term storage: While jars are hot, wipe rims clean, add lids, and screw on rings until fingertip-tight; do not overtighten. Return pot of water with canning rack to boil. Lower jars into water, cover, bring to boil, then start timer. Cooking time will depend on your altitude: Boil 10 minutes for up to 1,000 feet, 15 minutes for 1,001 to 3,000 feet, 20 minutes for 3,001 to 6,000 feet, and 25 minutes for 6,001 to 8,000 feet. Turn off heat and let jars sit in pot for 5 minutes. Remove jars from pot and let cool for 24 hours. Remove rings, check seal, and clean rims. (Sealed jars can be stored for up to 1 year.)