Combine kosher salt, black peppercorns, cumin seeds, fennel seeds, and crushed red pepper in a small skillet. Cook over medium, stirring often, until lightly toasted and fragrant, about 2 minutes. Transfer to plate; let cool. With a mortar and pestle or spice grinder, grind to a coarse powder. Add chopped fresh rosemary, garlic powder, and paprika. Stir well to combine.

Preheat oven to 450°F. Toss together chicken thighs, radishes, olive oil, and 2 tablespoons of the spice blend on a large rimmed baking sheet until evenly coated (Reserve the remaining spice blend for another use). Spread in a single layer. Roast in preheated oven until an instant-read thermometer inserted in thickest portion of chicken registers 160°F, about 30 minutes. Divide arugula evenly among 6 plates, and top with chicken, radishes, and pan juices. Sprinkle with salt to taste.