Step 1: Peel the red onions and slice them as thinly as possible. A mandoline slicer is recommended for uniform slices. Loosely pack the sliced red onions into a clean, heat-proof pint-sized jar.
Step 2: In a small saucepan, combine the water, vinegar, sugar, and salt. Add any optional spices like peppercorns, bay leaves, or red pepper flakes to the saucepan.
Step 3: Bring the brine mixture to a gentle boil over medium heat, stirring until the sugar and salt are completely dissolved. This should take about 1-2 minutes.
Step 4: Carefully pour the hot brine over the packed red onions in the jar, ensuring all the onions are completely submerged. If they float, gently press them down with a spoon.
Step 5: Let the jar cool down to room temperature on your countertop (about 30-60 minutes). Once cool, secure the lid tightly and transfer the jar to the refrigerator. The pickled red onions will be ready to eat in as little as 30 minutes, but are best after a few hours or overnight. Store in the refrigerator for up to 3 weeks.