Step 1

Use a mandoline or sharp knife to thinly slice the radishes into about ⅛-inch-thick circles. Place the radishes, shallots, and dill, if using, in a 16-ounce glass jar with a tight-fitting lid.

Step 2

In a small saucepan, combine the rice vinegar, sugar, salt, peppercorns, mustard seeds, crushed red pepper, and ¼ cup water. Bring the mixture to a boil over medium heat.

Step 3

Carefully pour the vinegar mixture over the radishes. Leave uncovered and allow to cool to room temperature, about 1 hour. Cover and refrigerate for at least 2 hours and up to 2 weeks.