In a large mason jar or sealable container combine vinegar, sugar, salt, and peppercorns.
Bring water to a boil. Place sliced red onion in a colander and then pour boiling water on top to blanch the onions. Transfer onions to the container with pickling liquid, seal, and let sit for at least 30 minutes.
Use the pickled onions on your favourite salads and sandwiches and store the rest in the refrigerator for 3-4 weeks.