Combine flour and 1/2 cup milk in a medium saucepan; stir until smooth. Stir in remaining 1 1/2 cups milk, milk powder, and margarine. Cook over medium heat, stirring constantly with a wire whisk, until mixture is thickened and bubbly. Stir in salt, pepper, and nutmeg.
Sprinkle garlic in a 13- x 9- x 2-inch baking dish coated with cooking spray. Arrange half of onion rings over garlic; top with half of potato slices. Arrange artichoke hearts over potato. Repeat layers with remaining onion and potato slices. Pour sauce mixture over potato slices, and sprinkle with Parmesan cheese. Bake, uncovered, at 350° for 40 minutes. Sprinkle evenly with Monterey Jack cheese. Bake an additional 20 minutes or until potato is tender. Let stand 10 minutes before serving.