Step 1: In a large bowl, combine the grated zucchini, diced onions, and diced bell peppers. Sprinkle with salt and toss well. Let sit for 2-3 hours (or overnight in the refrigerator) to draw out excess moisture.
Step 2: After resting, transfer the vegetable mixture to a colander and press firmly to squeeze out as much liquid as possible. You can also use a clean kitchen towel or cheesecloth to wring out the moisture. This step is crucial for preventing watery relish.
Step 3: In a large, non-reactive pot (like stainless steel), combine the apple cider vinegar, granulated sugar, mustard seeds, celery seeds, and turmeric. Bring the mixture to a boil over medium-high heat, stirring until the sugar dissolves.
Step 4: Add the drained vegetable mixture to the simmering brine. Stir well to combine. Bring the mixture back to a gentle boil, then reduce the heat to a simmer.
Step 5: Cook for 30-40 minutes, stirring occasionally, until the vegetables are tender-crisp and the relish has thickened to your desired consistency. The liquid should reduce significantly, and the relish should appear glossy.
Step 6: Ladle the hot relish into clean, sterilized pint or half-pint canning jars, leaving 1/2 inch of headspace. Remove any air bubbles. Wipe jar rims clean with a damp cloth, center lids, and apply bands finger-tight.
Step 7: Process jars in a boiling water bath for 10-15 minutes (adjusting for altitude if necessary). Carefully remove jars and let cool undisturbed on a towel for 12-24 hours. Check seals; lids should be concave and not flex. Store properly sealed jars in a cool, dark place for up to 1 year. Refrigerate unsealed jars and consume within a few weeks. Once opened, refrigerate and consume within 2-3 weeks.