Melt the butter in a large pot over medium heat, and cook until it turns nut-brown. Add the leeks and cook until slightly softened, about 2 minutes. Add the garlic and saute briefly to release its fragrance.
Add the squash and apples, raise the heat to high, and cook, stirring, until the vegetables begin to caramelize, about 5 minutes. Stir in the Toasted Spice Rub and cook briefly to toast it, about 1 minute.
Add the stock or broth-water mixture, bring to a simmer, and partially cover. Adjust the heat to maintain a gentle simmer and cook until the squash and apples are tender, about 40 minutes.
Transfer in batches to a blender or food processor and blend until smooth. Return to the pot, reheat to serving temperature, and season with salt.
Divide the soup among warmed bowls and garnish each portion with some of the walnuts, if using. Serve immediately.
Toasted Spice Rub
1/4 cup fennel seeds
1 tablespoon coriander seeds
1 tablespoon black peppercorns
1 1/2 teaspoon crushed red pepper flakes
1/4 cup pure California chili powder (about 1 ounce)
2 tablespoon kosher salt
2 tablespoon ground cinnamon
In a small, heavy pan, over medium heat, combine the fennel seeds, coriander seeds and peppercorns. When the fennel turns light brown, work quickly. Turn on the exhaust fan, add the red pepper flakes, and toss, toss, toss, always under the fan. Immediately turn the spice mixture out onto a plate to cool. Put in a blender with the chili powder, salt, and cinnamon and blend until the spices are evenly ground. If you have a small spice mill or a coffee grinder dedicated to grinding spices, grind only the fennel, coriander, pepper and chili flakes. Pour into a bowl and toss with the remaining ingredients.
Yield: about 1 cup