Wash the radishes thoroughly and trim off the stems and roots. Using a mandoline slicer or sharp knife, slice the radishes into thin 1/8-inch rounds—this consistency is crucial for even pickling and quick flavor absorption. While you're preparing the radishes, sterilize your jars by rinsing them with hot water or running them through the dishwasher. Once sliced, pack the radishes tightly into your clean jars, filling them about three-quarters full.

In a medium saucepan, combine the white vinegar, water, sugar, and coarse kosher salt. Bring the mixture to a boil over medium-high heat, then reduce to medium and stir constantly for about 1 minute until all the sugar and salt are completely dissolved. I like to taste the brine at this point—if it feels too sharp, I know the radishes will soften that acidity as they pickle. Add the smashed garlic cloves, peppercorns, mustard seeds, and red pepper flakes to the hot brine, stirring gently to distribute the spices evenly.

Carefully pour the hot pickling liquid from Step 2 over the radishes in the jars, ensuring all the radish slices are fully submerged and the spices are distributed throughout the jars. Let the jars sit at room temperature for about 20-30 minutes to begin cooling. I find that letting them cool slightly before refrigerating helps the radishes maintain a bit more crunch rather than becoming too soft.

Once the jars have cooled to room temperature, transfer them to the refrigerator. The radishes will be ready to eat after just 1 hour, though they'll develop deeper, more complex flavors if left overnight. The longer they sit, the more tender they become while still retaining a pleasant snap. They'll keep refrigerated for up to 2 weeks.