Slice the onion into 1/8 inch thin slices. (This step is easiest done using a mandolin slicer). Stuff the onions in a canning jar or a glass container.
1 large red onion
In a separate cup whisk together the hot water with the sugar and salt until dissolved. Stir in the apple cider vinegar.
1 cup hot water, 1/2 cup +1 tbsp apple cider vinegar, 1 tbsp granulated sugar, 1.5 tsp sea salt
Pour the pickling mixture over the sliced onions, cover with a lid and allow to sit on the kitchen counter until completely cool. Transfer to the refrigerator overnight. Serve the next day.
Notes
Recipe Variations
Peppercorns - Most traditional pickling recipes contain some type of black or pink peppercorns. Feel free to sprinkle about 1 tablespoon in the jar.
Herbs - Fresh dill and dried bay leaves are another classic addition to brines. They would both be lovely here if you have them available.
Citrus -If you like you can entirely omit the vinegar and replace it with a fresh citrus juice made from a combinations of 1/3 cup orange juice + 1/4 cup freshly squeezed lime juice. Adjust to your taste from here. You can go for plain lime juice if you prefer an extra zesty brine.