Try these sweet and easy refrigerator zucchini pickles perfect on hamburgers, but also great all on their own. Make them ahead of time so you're ready for the next barbeque you host or attend with this unique and palatable dish.
Put the cider vinegar, sugar, and salt in a small saucepan. Put over medium-high heat and bring to a boil. While that's heating up, crush the mustard seeds—you can use a mortar and pestle if you have them, or put the seeds in a resealable plastic bag and pound them gently with a rolling pin, flat meat tenderizer, or a small frying pan. Add the crushed mustard seeds, dry mustard, and turmeric to the vinegar mixture.
Once the mixture boils, reduce the heat to low so the mixture simmers gently. Let it simmer for 3 minutes. Remove the pan from the heat, pour the mixture into a large bowl, and let cool until just warm to the touch.
Cut the zucchini into slices 1/8- to 1/16-inch thick. A mandoline, if you have one, is handy for this, but a sharp knife and steady hand work just as well. You can cut the zucchini crosswise or lengthwise.
Trim the root end of the onion. Cut it in half lengthwise and remove the peel. Set the 2 halves on a cutting board, cut-side-down, and thinly slice the onion.
When the brine is cooled, add the zucchini and onion slices. Stir to coat the vegetables completely with the brine.
Divide the zucchini and onion evenly between the 3-pint jars, leaving about 1 inch of space between the tops of the vegetables and the tops of the jars. Add enough brine in each jar to cover the vegetables, leaving at least 1/2 inch space between the top of the liquid and the tops of the jars.
Use a paper towel to wipe the rims and edges of the jars clean. Screw on the lids. Put in the fridge and let chill for at least 1 week and up to 6 months.