Preheat oven to 425°. On a baking sheet, toss radishes with oil and syrup; season with salt and pepper. Roast until radishes are just tender and greens are crisp, about 15 minutes.
Meanwhile, in a food processor, blend the feta and 2 tablespoons water until smooth and spreadable. Add up to 2 tablespoons more water if necessary, 1 tablespoon at a time. Add dill, and pulse to combine. Season with pepper.
Spread the whipped feta on the bottom of a platter, and arrange the roasted radishes over the feta. Top with dill sprigs and nigella seeds.
Recipe by Laura Rege.