Melt margarine in a heavy saucepan over low heat; add flour, stirring until smooth. Cook, stirring constantly, 1 minute. Gradually add cream and milk; cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Stir in salt, pepper, and nutmeg.
Sprinkle blue cheese and garlic in a lightly greased 13- x 9- x 2-inch baking dish. Arrange half of onion rings over blue cheese and garlic; top with half of potato slices. Arrange artichoke hearts over potato slices. Pour half of sauce mixture over artichoke hearts. Repeat layers with remaining onion, potato slices, and sauce mixture. Bake, covered, at 350° for 1 hour. Sprinkle evenly with Cheddar cheese. Bake, uncovered, an additional 15 minutes or until potato is tender. Let stand 10 minutes before serving.