Trim all the squash at both ends and cut into quarters lengthwise. Scoop out seeds with a spoon and slice the squash into 1/2" slices. Heat the butter in a large heavy saucepan over medium heat until melted and foaming. Add onion and squash stirring and coating with butter. Add 2 teaspoons of salt, 1/4 teaspoon pepper and thyme. Reduce heat to low and cover. Cook for 20 minutes, stirring occasionally. In a separate saucepan, heat chicken stock to boiling and then remove from heat. When squash is tender, add chicken stock and simmer partially covered for another 15 minutes. Remove from heat and puree in a food processor or blender in batches until all is pureed. Be careful when workinig with hot liquid. Strain through a sieve into another pot and return to the stove. Add cream. Heat the soup to a simmer and season to taste with salt and pepper. Ladle into serving bowls and grate a few grindings of fresh nutmeg on top of each serving.