Bring a pot of water to a boil. Place the sliced red onions in a heat proof bowl. Pour the boiling water over the onions. Let sit for one minute. Strain. Discard the water. Place the onions in a pint sized mason jar.
Combine the 3/4 cup water, vinegar and pickling salt. Whisk to dissolve the salt. Pour over the onions in the jar. Store in the refrigerator for up to two weeks. The onions will be ready to eat after a couple of hours, but best overnight.