Thinly slice the red onions and divide them evenly among 2–3 sterilized jars.
In a saucepan, combine the white vinegar, water, cane sugar, and sea salt. Heat this mixture over medium heat while stirring continuously until everything is fully dissolved.
Once dissolved, remove the saucepan from heat and let the brine cool slightly until it reaches a warm temperature.
Pour the warm brine over the sliced onions in your jars, ensuring they are completely submerged.
Allow the jars to cool at room temperature without sealing them initially. Once they reach room temperature, seal them tightly and place them in the refrigerator.
Your pickled onions will be ready to eat after just one hour but taste even better after resting overnight in the fridge.