1. Melt butter in a large saucepan over medium heat. Add onion; cook, stirring occasionally, until it begins to soften, about 4 minutes. Add squash, and cook, stirring occasionally, until soft, about 10 minutes.

2. Add apples, salt, cumin, coriander, ginger, cayenne, black pepper, stock, and the water (just enough to cover). Bring to a boil. Reduce to a simmer, and cook until vegetables are very soft, about 30 minutes.

3. Puree in batches in a food processor or blender until smooth, and return to saucepan. Heat over low, thinning with more water if necessary. To serve, ladle into shallow bowls; garnish with diced apples, jalapeno slices, and sour cream if desired.