Melt the butter in a large pot over moderate heat and cook until it turns nut brown. Add the leeks and sauté until slightly softened, about 5 minutes. Add the garlic and sauté 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 spice rub and cook briefly to toast it. Add the stock or the broth-water mixture, bring to a simmer, and cover partially. 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. |