Prepare the radishes: Rinse thoroughly and trim tops and roots. Slice thin (using a mandoline if available) for red radishes, or cut daikon into half-moons or batons approximately ¼-inch thick.
Make the brine: Combine vinegar, water, sugar, and salt in a saucepan over medium heat. Stir until sugar and salt completely dissolve, about 2-3 minutes. Remove from heat and allow to cool to room temperature.
Pack your jars: Distribute radish slices among clean glass jars. Add peppercorns, mustard seeds, and any optional aromatics.
Pour the brine: Once cooled to room temperature, pour brine over radishes until fully submerged. Seal jars with lids.
Refrigerate and serve: Thin-sliced red radishes are ready in 1 hour. Daikon and thicker slices benefit from 24 hours. Flavors continue developing for 3-4 days. Keeps refrigerated for 2-3 weeks.