Yes, it is possible to make these without a stove. Just microwave the vinegar, water, sugar, and salt in a microwave-safe container until it’s hot and the sugar and salt have fully dissolved. Then proceed to carefully pour the vinegar solution over the onions the same way. The vinegar solution may not get as hot or stay as hot as it would on the stovetop, but this should not change the flavor of the pickled red onions too much.