Preheat the oven to 400°F. Spread the squash cubes on a rimmed baking sheet (they will crowd each other; that’s ok).
In a large soup pot (which you will use later) melt the butter with the coriander and cumin, and chili powder, stirring until you can smell the spices.
Pour the butter over the squash and toss to coat the squash with the spiced butter. Season with salt and pepper. Roast for 40 minutes until the squash is golden brown and very tender.
While the squash is roasting, heat the olive oil in the pan you melted the butter in over medium low heat and cook the leeks, partially covered, for about 30 minutes, stirring occasionally until very soft but not browned. Add a splash of water as needed to keep the leeks from browning.
Transfer the leek mixture to a food processor or blender and add 1 cup of the broth. Puree until fairly smooth (there will be flecks of leeks). Return it to the pot. Add the cooked squash with 1 more cup of the broth and puree until smooth. Add this mixture to the leek-broth mixture, add the remaining cup of broth, and stir to combine. Bring to a simmer over medium heat. Stir in the lemon or lime juice and the cream and cook, stirring frequently, for another minute until hot but not simmering. Taste and adjust the seasonings.
Serve hot with the pepitas on top for garnish, if using.