Peel the garlic and smash it carefully with the flat side of a chef's knife or with a small pot.
Cut the ends off the radishes and thinly slice them.
Halve the cucumbers lengthwise and thinly cut crosswise.
In a small sauce pot, combine the smashed garlic, vinegar, sugar, mustard seeds and ¼ cup of water.
Bring to a boil and cook for a couple of minutes while stirring until the sugar dissolves. Remove from heat.
In a small bowl, add the radishes and cucumbers and cover with the marinade.
Let sit for at least 10 minutes. Taste and adjust seasoning with salt and pepper.
Serve as garnish.