Pack the sliced onion into a pint sized jar. It should be very tightly packed or you will not have enough brine.
Combine the water, vinegar, sugar, and salt. Heat and stir to dissolve the sugar and salt using either the stove top or the microwave.
Pour the brine over the onions in the jar. Let rest a minimum of 30 minutes for very thinly sliced onions, and longer for more thickly sliced onions.
Refrigerate the pickled onions after 30 minutes, and up to 3 weeks.
