1. Melt butter in a large saucepan over medium heat. Add onion, and cook until it begins to soften, about 4 minutes. Add squash, and cook, stirring occasionally, until it softens, about 10 minutes. Add apples, salt, cumin, coriander, ginger, cayenne, pepper, stock, and 2 1/2 cups water (just enough to cover). Bring to a boil.

2. Reduce to a simmer, and cook until vegetables are very soft, about 30 minutes.

3. Puree in batches in the bowl of a food processor or the jar of a blender until smooth, and return to saucepan over low heat. Thin with additional water if necessary. Serve hot, garnished with chopped apples, jalapeno, and sour cream, if desired.