Thinly slice the red onions as evenly as you can. Divide the sliced onions between 2 (16-ounce) jars or 3 (10-ounce) jars. If you are using garlic and peppercorns, add them into each jar with the onions.
In a medium saucepan over medium heat, combine the white vinegar, water, cane sugar, and sea salt. Stir gently and heat just until the sugar and salt dissolve, about 1 minute.
Remove the saucepan from the heat and let the liquid cool slightly. Carefully pour the warm liquid over the onions in the jars, making sure they are fully covered.
Let the jars sit at room temperature until cooled. Then place them in the refrigerator.
The onions are ready once they turn bright pink and tender. Very thin slices will be ready in about 1 hour. Thicker slices may need to sit overnight. They will keep in the fridge for up to 2 weeks.