Carefully slice the red onion very thinly using a sharp knife or a mandoline. Aim for uniform slices about 1/16 to 1/8 inch thick. Pack the sliced onions tightly into your clean, heat-proof pint-sized glass jar.

In a small saucepan, combine the water, apple cider vinegar, granulated sugar, and kosher salt. If using, add the black peppercorns, bay leaf, and sliced garlic to the saucepan now.

Place the saucepan over medium-high heat and bring the mixture to a rolling boil, stirring occasionally to dissolve the sugar and salt. Once boiling, remove immediately from heat.

Carefully pour the hot brine over the packed red onions in the jar, making sure the onions are fully submerged. You may need to press them down gently with a spoon. Ensure any optional aromatics are also in the jar.

Seal the jar with its lid. Allow the jar to cool on the counter for about 20-30 minutes, or until it reaches room temperature. The onions will soften and turn a vibrant pink color as they pickle.

Once cooled to room temperature, transfer the jar to the refrigerator. For best flavor, let them chill for at least 1-2 hours before serving, though they can be eaten after cooling. The longer they sit, the more pronounced the pickled flavor will become.