Preheat your oven to 220°C (430°F) and line an oven tray with baking paper.
Pour the olive oil into a small bowl and stir in the salt and cumin until well combined; spread the pumpkin pieces out on the tray, ensuring they’re spaced apart; use a pastry brush to coat the pumpkin pieces with the spiced olive oil; then slide the tray into the oven onto the top shelf.
Roast the pumpkin pieces for 15 minutes or so until they start to soften, begin to brown and even char a little; slide the tray out of the oven; use tongs to turn the pieces over; brush any leftover spiced olive oil onto the pieces then return the tray to the oven for another 10 minutes or so.
Make the yogurt sauce after putting the pumpkin on to roast so the flavours can meld together: in a small bowl, stir the lemon juice, garlic powder, salt and white pepper together; pour in the yogurt and olive oil and stir vigorously so everything is well combined, then set aside.
In a small fry pan over medium heat, toast the whole cashews, moving the pan continually so the cashews don’t burn; as soon as they start to colour remove them and slide them into a small dish, then do the same to the pistachios.
When the pumpkin is roasted to your liking, transfer the pieces to a serving plate, douse the pumpkin pieces with the yogurt sauce, then sprinkle on each of the dukkah, toasted nuts, sumac, and fresh dill, and serve immediately.