Preheat oven to 425 F.
Wash and trim the radishes and cut into bite size pieces (so that all pieces are roughly the same size and roast uniformly).
Toss radishes, directly on rimmed baking sheet, olive oil to coat and season with sumac, salt and pepper. Arrange in an even layer (don’t overcrowd the pan), and roast in oven, stirring occasionally, until radishes are tender and lightly browned, about 20-30 minutes.
Meanwhile, to make the dukkah, toast cumin and coriander seeds briefly in a small dry skillet over medium heat until fragrant and slightly golden, about 2-3 minutes. Transfer to bowl of a food processor and let cool slightly.
Blitz food processor with cumin seeds and coriander seeds briefly, until they just start to break down. Add the roasted pistachios and blitz a few time more (make sure not to over-blend – we’re going for a chunky texture and not a nut butter).
Pour the spiced pistachio mixture into a small bowl and add the salt and sumac.
Serve radishes warm, sprinkled with dukkah and a squeeze of lemon juice. Serve with additional dukkah in a small bowl alongside for more sprinkling.