Chop and salt: Finely chop cucumbers, onions, and bell peppers. In a large mixing bowl, combine chopped vegetables with pickling salt. Mix well.
Drain: Cover the bowl and let stand for 2 hours. Then, rinse the mixture in a colander under cold water and allow to drain completely. It draws out water from the vegetables and keeps your relish crunchy. Skipping it might give you a softer relish than you're hoping for.
Sterilize jars: Heat jars in simmering water until ready for use. Do not boil. Wash lids in warm, soapy water and set aside with bands.
Combine vinegar and spices: In a large pot, mix the sugar, apple cider vinegar, mustard seeds, celery seeds, and turmeric. Bring to a boil, stirring until the sugar dissolves.
Add vegetables: Add the drained vegetable mixture to the vinegar solution. Return to a boil and simmer for 10 minutes.
Fill jars: Pack the hot relish into hot jars, leaving ½ inch headspace. Remove any air bubbles. Wipe the rim of the jars with a clean, damp cloth. Place the lid on the jar and secure with the band until fingertip tight.
Process: Place jars in the waterbath canner, making sure they are covered by at least 1 inch of water. Bring to a rolling boil and process for 10 minutes, adjusting for altitude. Remove from the canner and allow to cool undisturbed.
Check seals: After cooling, check that lids are properly sealed by pressing the center of the lid. It should not flex. Store in a cool, dark place.