Cook the corn on the cob using your preferred method until just tender. I’m partial to steaming it over high heat in about 1 inch of water in a large, covered pot. I usually move the corn around in the pot once during cooking and check for doneness by popping a kernel off of the cob using a butter knife and tasting it. Corn takes about 5 to 7 minutes to steam.
While the corn is cooking, make the pickled onions. Mix together the vinegar, sugar, and 1 teaspoon of the salt in a small bowl, stirring until the solids dissolve. Add the onion slices and submerge completely in the vinegar. Let that stand for at least 20 minutes.