Step 1: Wash and prepare the radishes. Thoroughly wash the radishes and trim off the green tops and root ends. Using a mandoline or a sharp knife, slice the radishes thinly and uniformly, aiming for about 1/16 to 1/8 inch thick.
Step 2: Pack the jars. Divide the sliced radishes evenly between two clean, heat-proof pint-sized glass jars. Add 2 thinly sliced garlic cloves, 1/2 teaspoon of whole black peppercorns, and 1/8 - 1/4 teaspoon of red pepper flakes (if using) to each jar.
Step 3: Prepare the pickling brine. In a small saucepan, combine 1 cup of water, 1 cup of vinegar, 1 tablespoon of granulated sugar, and 1 teaspoon of sea salt. Bring the mixture to a gentle boil over medium-high heat, stirring until the sugar and salt are completely dissolved. Do not overboil.
Step 4: Pour the hot brine. Carefully pour the hot pickling brine over the radishes in each jar, ensuring all radishes are fully submerged. Tap the jars gently on the counter to release any trapped air bubbles.
Step 5: Cool and seal. Allow the jars to cool completely to room temperature on your countertop. Once cool, secure the lids tightly.
Step 6: Refrigerate and enjoy. Transfer the sealed jars to the refrigerator. The pickled radishes can be enjoyed after 30 minutes, but their flavor will be best after at least 2 hours, and even better the next day. They will keep well in the refrigerator for up to 2-3 weeks.