To make the buckwheat: Preheat the oven to 400˚F. Spread out the buckwheat on a baking sheet. Toast in the oven until the buckwheat deepens in color and has a nutty aroma, about 8 minutes. Let cool, then use a mortar and pestle to crush the buckwheat into a texture resembling coarse sand.
To prep the salad ingredients: While the buckwheat is toasting, thinly slice the radish into half-moons and transfer to a small bowl of ice water until ready to use. Place the lettuce in a large serving bowl.
To make the dressing: Place the garlic in a blender or food processor and pulse it into small pieces. Add the remaining dressing ingredients and blend until smooth.
To assemble the salad: Drain and pat dry the radish slices. Add a handful to the bowl of lettuce along with ⅓ cup of the dressing. Reserve the remaining dressing for another use. Toss until everything is well coated. Top with the remaining radish and a hefty sprinkle of the crushed buckwheat and some black pepper.