How to Make It
In a small skillet, toast the nigella and mustard seeds over moderate heat until fragrant, about 1 minute. Transfer the seeds to a plate to cool completely.
In a small bowl, combine the sour cream with the vinegar and cumin and season with salt and pepper.
Arrange the radishes on a large plate or platter. Drizzle with the dressing. Garnish with the toasted seeds and serve.
Variation To turn this into a radish slaw, cut the radishes into matchsticks (we like using a julienne peeler or a mandoline to do this) and toss with the sour cream dressing and the toasted nigella and mustard seeds.