1Heat oven to 350°F. Spray 3-quart glass baking dish with cooking spray.
2In 2-quart saucepan, melt butter over medium heat. Cook and stir garlic in butter 1 minute. Stir in flour with whisk until smooth. Gradually add half-and-half and cream. Heat to boiling. Reduce heat to low; cook about 5 minutes, stirring frequently, until sauce is slightly thick. Stir in salt, pepper and nutmeg. Slice potatoes into 1/8-inch-thick slices.
3In large bowl, toss potatoes with cream sauce. Layer half of the potatoes in baking dish; top with green onions. Sprinkle with half each of the Cheddar and Parmesan cheeses. Repeat layers, using up potatoes and cheese.
4Bake 55 to 65 minutes or until potatoes are tender and top is golden brown. Let stand 5 to 10 minutes before serving. Cover and refrigerate any remaining potatoes.