Warning: Pickled watermelon radishes are not like traditional pickled radishes. Upon fermentation in the fridge, the radish slices undergo a dramatic transformation, releasing a smell that might make you shudder. That said, the slices themselves taste much milder than they smell, and with a little breathing time, their pungency tempers. I find them addictive -- they are the perfect condiment in a bánh mì sandwich or in a whole grain bowl or in anything that needs that acidic bite. These pickled radishes will last indefinitely stored in the fridge but are best eaten within 3 months.
Wash your watermelon radishes well, making sure to remove any dirt. Using a mandoline or a sharp knife, thinly slice your watermelon radishes into discs, then place in a clean canning jar.
In a non-reactive saucepan, bring the water, vinegar, salt, and sugar to a boil, and simmer for one minute or until the sugar and salt are dissolved. Remove from heat and add the garlic and peppercorns. Pour the hot liquid including the garlic and peppercorns over the radishes.
Let cool to room temperature, then cover and refrigerate.