Step 1: Prepare the onion. Thinly slice the red onion using a sharp knife or a mandoline. Aim for uniform, thin slices for best results.
Step 2: Pack the jar. Tightly pack the sliced red onion into a clean, heat-proof pint-sized jar or a glass container with a lid. If using, add sliced garlic, peppercorns, and red pepper flakes to the jar.
Step 3: Combine pickling liquid ingredients. In a small saucepan, combine the apple cider vinegar, water, granulated sugar, and salt.
Step 4: Heat the liquid. Bring the pickling liquid to a boil over medium-high heat, stirring until the sugar and salt are completely dissolved. This usually takes about 2-3 minutes.
Step 5: Pour over onions. Carefully pour the hot pickling liquid over the sliced onions in the jar, ensuring all the onions are fully submerged. You may need to press them down gently with a spoon.
Step 6: Cool and seal. Let the jar cool to room temperature on your countertop. Once cooled, secure the lid tightly.
Step 7: Chill and enjoy. Transfer the jar to the refrigerator. The pickled onions will be ready to enjoy in as little as 30 minutes, but their flavor will deepen and become more vibrant after a few hours or overnight. Store in the refrigerator for up to 2-3 weeks.