In a heavy large saucepan, melt butter over medium heat. Add onions and cook until tender about 5 minutes. Stir in thyme, stewed tomatoes, and pumpkin puree and bring to a simmer. Stir in warmed stock and return to a simmer and cook for 20 minutes. Puree soup in 3 batches in a blender. Season with nutmeg, salt and pepper. Stir in cream and return to simmer. Serve with sliced scallions.