Melt the butter in a large saucepan or Dutch oven and saute' the onion, garlic, scallions, and ham for 5 minutes. Add the mushrooms, reduce the heat to medium and cook for 2 minutes. Blend in the flour and cook, stirring, for 4 minutes, then add the Worcestershire, beef stock, thyme, and bay leaf. Simmer until the sauce thickens, about 1 hour. Before serving, remove the bay leaf and add the parsley. Season with salt and pepper to taste. Yields 3 cups.