Trim ends off onion and take off outer peel. Thinly slice and place in a clean jar.
Whisk together hot water, vinegar, maple syrup and salt.
Pour vinegar mixture over the onions. Push onions down to be sure all slices are covered in liquid. Cover to make an airtight seal.
Pickled red onions will be ready to enjoy in 30 minutes. They will develop a stronger flavor over time.
Store in the refrigerator for up to 2 weeks.
Notes
Vinegar: Use apple cider vinegar, distilled white vinegar or a combination of both.Onions: Use red onions, large shallots, yellow onions, or spring green onions.