Step 1Trim and cut the zucchini, onions and peppers into a one-fourth-inch dice and toss in a colander with the 3 tablespoons kosher salt. Set the vegetables aside for 2 hours to allow the salt to draw water from them and drain. After 2 hours, rinse the vegetables by pouring a kettle of boiling water over them; drain well.
Step 2Heat the cider vinegar, sugar, pickling salt, honey, mustard seed, celery seed, turmeric, curry powder, ground peppercorns and coriander, dried chile pepper and garlic in a large saucepan or small pot over medium-high heat until the sugar and coarse salt are dissolved, about 2 to 3 minutes. Stir in the vegetables and simmer over medium heat until the vegetables are translucent and yielding, about 20 to 25 minutes. Remove from the heat and discard the garlic.
Step 3Pack the hot relish into jars and can according to the jar manufacturer's instructions, or cool and refrigerate. Set the relish aside for a week to give the flavors time to blend and mellow.