Melt the butter in a large heavy saucepan. Add the leeks, celery and garlic and cook over moderate heat, stirring occasionally, until softened, 7 to 8 minutes. Add the stock, squash, tomato paste and bay leaf. Season with salt and pepper and simmer over moderately low heat until all the vegetables are tender, about 25 minutes. Discard the bay leaf and add the parsley.