Preheat oven to 425° F. Spray a 9 X 5-inch ovenproof loaf pan with vegetable oil spray; set aside.
To make the loaf, heat oil in a large, nonstick skillet at medium setting. Add onion and sauté, stirring frequently, about 4 minutes. Add garlic and continue to sauté, stirring constantly, 1 minute more, or until onion is soft.
Mix bread crumbs and buttermilk in a large bowl and let soak 5 minutes. Add sautéed onion and garlic, egg white, pepper, nutmeg, ginger, allspice and dill. Use your hands to lightly mix. Add ground beef and chicken. Mix well to evenly distribute ingredients. Lightly pat into prepared loaf pan. Place in oven and reduce temperature to 350° F. Bake 30 minutes.
While meatloaf is cooking, make the sauce. In heavy saucepan, melt margarine over medium heat. Remove pan from heat and blend in flour. Return to heat and gradually add beef stock, sherry and milk. Stir constantly with a wire whisk until fully blended and mixture thickens, about 6 to 8 minutes. Remove from heat and blend in remaining ingredients.
Remove meatloaf from oven and pour off fat. Pour sauce over meatloaf, cover with foil and return to oven. Continue to cook 15 minutes more or until done. Serve hot.