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.
Use an immersion blender, regular blender or food processor to purée the soup to your desired consistency. I like it a little chunky.
Season with salt to taste (take into account your broth might contain salt).