Cut squash into 1-inch chunks.
In large pot melt butter and add onion and cook until translucent, about 8 minutes.
Add squash and stock and bring to a simmer and cook until squash is tender, about 15 to 20 minutes.
Remove squash chunks with slotted spoon and place in a blender and puree then return blended squash to pot.
Stir and season with nutmeg, salt, and pepper. Serve.