Step 1: Peel and thinly slice the red onion. For best results, use a mandoline for uniform, super-thin slices.
Step 2: Loosely pack the thinly sliced red onions into a clean, heat-proof glass jar. Don't press them down too tightly.
Step 3: In a small saucepan, combine the water, vinegar, sugar, and salt. Add any optional flavorings like peppercorns, bay leaf, or red pepper flakes to the saucepan now.
Step 4: Place the saucepan over medium-high heat and bring the mixture to a gentle boil, stirring occasionally, until the sugar and salt are completely dissolved (about 2-3 minutes). Remove from heat.
Step 5: Carefully pour the hot brine over the packed onions in the jar, ensuring all onion slices are fully submerged. Gently press down with a spoon if needed.
Step 6: Let the jar cool down to room temperature on your counter. As it cools, the onions will begin to soften and turn a beautiful pink color. Once cooled, seal the jar tightly with a lid.
Step 7: Transfer the jar of pickled onions to the refrigerator. They are good to eat after 30 minutes, but develop best flavor after a few hours or overnight. Store in the refrigerator for up to 2-3 weeks.