Heat water in a saucepan.
Using a mandolin, slice red onions as thin as possible. If you don't have a mandolin, slice them with a knife.
Put all the sliced onions in a jar.
In a bowl or Pyrex measuring cup, combine water, vinegar, sugar, and salt, and stir until solids are dissolved. Pour the pickling mixture into the jar and make sure the onions are completely submerged.
Optionally add spices such as cracked or uncracked peppercorns, red pepper flakes, a smashed clove of garlic, thyme, oregano, or whatever you like.
Pickled onions will keep in the fridge for 2 weeks.