Heat butter in large heavy pot over medium heat. Add onions, leek, garlic, apple, thyme, and bay leaf and cook until onions start to throw off liquid, about 5 minutes. Add squash, sweet potatoes, broth, allspice, cloves, and cinnamon. Heat to boiling. Reduce heat and simmer uncovered until squash is very tender, about 30 minutes.
Remove bay leaf and cinnamon stick; let soup cool slightly. Transfer to blender or food processor and puree until very smooth. Strain back into pot.
Mix half-and-half, cream, and sour cream. Heat soup over low heat and gradually whisk in cream mixture. Stir in peas. Gently heat until peas are hot, but do not let soup boil. Season with salt and pepper.
Ladle soup into bowls. Sprinkle with parsley and scallions.