To begin, layer several slices of onions in the bottom of a pint jar. Wide mouthed jars work best.
On top of the radish slices layer some onion strips.
Repeat the layers until you have filled the jar. Pack the contents in tightly.
Combine the vinegar, sugar, and sea salt in a dish or another jar with a lid. Shake well to combine or whisk if using a dish.
Pour the liquid into the jar with the radishes.
Close the jar with a lid and refrigerate for at least 8 hours or overnight. Occasionally give the jar a shake to make sure everything is combined and pickling well.