Begin by peeling the red onions and slicing them as thinly as possible; a sharp knife or mandoline will yield the best results.

Divide the thinly sliced onions evenly among clean, sterilized jars, adding garlic and mixed peppercorns if desired.

In a small saucepan, combine the white vinegar, water, cane sugar, and sea salt to create the brine.

Carefully pour the warm brine over the sliced onions in the jars to ensure they are fully submerged.

Allow the jars to cool to room temperature without lids, then seal and refrigerate.

Let the pickled onions marinate for at least 1 hour before serving, ideally overnight.