Trim the ends of the zucchini and slice into 0.25-inch rounds; thinly slice the onion.
Toss the slices with kosher salt in a large nonreactive bowl to coat evenly; let stand 1 hour to draw out moisture, stirring once. Rinse well under cold water and drain thoroughly; spin or pat dry.
Combine apple cider vinegar (5% acidity), water, granulated sugar, pickling salt, mustard seeds, celery seeds, ground turmeric, black peppercorns, and garlic in a saucepan. Bring to a boil over medium-high heat, stirring to dissolve the sugar, then reduce heat and simmer 2 minutes until very fragrant.
Pack the drained vegetables tightly into two clean 1-quart jars, leaving 0.5-inch headspace. Divide the hot spices between the jars and ladle in hot brine to cover, tapping the jars to release air bubbles; top up so everything is submerged.
Wipe rims and apply lids. Cool at room temperature until just warm, 30–45 minutes, then refrigerate at least 24 hours before serving; keep refrigerated and use within 1 month.