Roast bell peppers over an open flame or in broiler, turning frequently, until skins charred and blistered. Remove skins, seeds, and stems; chop flesh into small pieces.

Place grated onions in a colander to drain. Combine with cabbage in a large bowl and toss with kosher salt; let sit for 1 hour.

Squeeze out liquid from onions and cabbage using a colander and cheesecloth.

Add onions, cabbage, and roasted peppers to a large pot. Stir in sugar, vinegar, turmeric, mustard seeds, and celery seeds. Bring to a boil, then reduce heat and simmer for 45 minutes.

Sterilize 20 8-ounce jars by boiling them in a canning pot. Rinse lids and prepare for sealing.

Fill sterilized jars with hot relish, remove air bubbles, wipe rims, and seal with lids and rings.