Step 1: Prepare the Radishes. Wash the radishes thoroughly and trim off the green tops and root ends. Using a sharp knife or a mandoline slicer, slice the radishes as thinly as possible (about 1/16 to 1/8 inch thick). Place the sliced radishes into your clean glass jar.
Step 2: Make the Brine. In a small saucepan, combine the water, vinegar, sugar, and salt. If using optional aromatics like garlic slices, peppercorns, or red pepper flakes, add them to the saucepan now. Bring the mixture to a boil over medium-high heat, stirring until the sugar and salt have completely dissolved. Once dissolved, remove from heat.
Step 3: Pour the Brine Over Radishes. Carefully pour the hot brine over the sliced radishes in the jar. Make sure the radishes are fully submerged in the liquid. If they aren't, gently press them down with a spoon or add a little extra hot water to top them off.
Step 4: Cool and Refrigerate. Allow the jar to cool completely to room temperature on your counter. Once cool, secure the lid tightly and transfer the jar to the refrigerator.
Step 5: Pickle and Enjoy. The radishes will be ready to eat in as little as 1-2 hours, but their flavor will deepen and become more pronounced after 24-48 hours. For the best flavor, let them sit in the fridge for at least a day.