Rinse radishes, then cut off ends and slice thinly.
Peel garlic cloves and cut in half.
Pack radishes and garlic cloves into a very clean pint jar.
Add water, vinegar, sugar, and salt to a small saucepan and place over medium heat. Cook for 2-3 minutes, whisking frequently, until salt and sugar have dissolved.
Remove from heat and stir in peppercorns and mustard seeds.
Pour brine over radishes, making sure everything is submerged. Spoon in any peppercorns or mustard seeds that might be floating in leftover liquid.
Let cool for 20 minutes, then move into the refrigerator. Chill for at least an hour before serving.
Notes
Storage Tips: Pickled radishes should be stored in their airtight container in the refrigerator, where they will keep for up to 2 weeks.Expert Tips:
Slice your radishes thinly for the best texture (softening a little but staying crisp). Use a mandolin if you want very uniform thin slices, or just a sharp knife.
The best way to sterilize a pint jar before using it to store pickles is just to run it through the dishwasher before using.