If you haven't roasted the pumpkin, peel, cube and roast in a moderate oven until soft (20-30 minutes).
While you wait, make the sauce: whisk tahini, water and lemon juice together until smooth and with the consistency of yoghurt or pouring cream. Add more water if needed. Season with salt and sumac.
Add pumpkin, lentils and broth to a small saucepan, bring to a boil.