Step 1Heat the oven to 375 degrees. Place the cinnamon stick, the allspice, cloves, star anise, fennel seeds, thyme, bay leaf and orange peel on a length of cheesecloth, and tie it up into a sachet. Set aside.
Step 2Place the butternut and acorn squashes cut side up on a baking sheet. Sprinkle with salt and pepper, and roast until tender, about 1 hour. Set aside.
Step 3While they're roasting, heat the olive oil in a large pot, add the onion, apple and banana, sprinkle with the sugar, and sweat them until caramelized, about 10 minutes.
Step 4Deglaze the pan by pouring in the orange juice, stirring and scraping the bottom with a wooden spoon to loosen any bits. Add the sweet potato, and stir to coat.
Step 5Pour in the milk and cream, add 4 tablespoons of the butter and the sachet. Bring to a gentle simmer, and poach the sweet potatoes over medium heat until they're very tender, about 45 minutes. Strain the sweet potatoes, reserving the poaching liquid. Discard the sachet.
Step 6Scoop the roasted squash out of its shells, and place the flesh in a potato ricer. Add the sweet potatoes and press through. You may need to do this in batches. Then pass the mixture through a fine sieve into a large saucepan.
Step 7Add enough poaching liquid to yield a silky puree, along with the remaining butter. Season with the sea salt and white pepper. May be prepared ahead up to this point, and reheated on low heat.