Step 1: Prepare the Vegetables. Wash and trim the ends of the zucchini. Using a food processor with a grating attachment or a box grater, grate the zucchini. Finely dice the onions and bell peppers. Place all grated and diced vegetables in a large non-reactive bowl.
Step 2: Salt and Drain. Sprinkle the salt over the vegetables and toss to combine. Let this mixture stand for at least 3-4 hours, or preferably overnight, in the refrigerator. This step draws out excess moisture. After soaking, thoroughly drain the vegetables in a colander, pressing out as much liquid as possible.
Step 3: Prepare the Brine. In a large non-reactive pot (stainless steel is ideal), combine the white vinegar, granulated sugar, celery seeds, turmeric, mustard seeds, and black pepper. Bring the mixture to a boil over medium-high heat, stirring until the sugar is completely dissolved. Reduce heat and let it simmer for 5 minutes to allow the flavors to meld.
Step 4: Cook the Relish. Add the drained vegetable mixture to the simmering brine. Bring the entire mixture back to a gentle boil, then reduce the heat to a simmer. Cook for 20-25 minutes, stirring occasionally, until the vegetables are tender-crisp and the relish has thickened slightly.
Step 5: Prepare Jars for Canning (Optional but Recommended). While the relish is cooking, sterilize your canning jars and lids according to standard canning procedures. Keep them hot until filling.
Step 6: Fill Jars. Carefully ladle the hot relish into the hot, sterilized jars, leaving 1/2 inch of headspace. Remove any air bubbles. Wipe the jar rims clean with a damp cloth.
Step 7: Seal Jars. Center the lids on the jars and apply the bands, tightening them to fingertip-tight.
Step 8: Process in a Water Bath. Place the filled jars into a boiling water canner, ensuring they are fully submerged. Bring water to a rolling boil and process pint jars for 10 minutes. After processing, turn off heat, remove canner lid, and let jars sit for 5 minutes before carefully removing them to a wire rack to cool for 12-24 hours. Check seals; refrigerate unsealed jars. Store sealed jars in a cool, dark place.