Heat oven to 350°F. Spray 1 1/2-quart casserole with nonstick cooking spray. Cook mostaccioli to desired doneness as directed on package. Drain.
Meanwhile, melt margarine in large saucepan over medium heat. Stir in flour, salt, pepper and nutmeg; cook and stir until bubbly. Gradually add half-and-half, stirring constantly. Cook until mixture boils and thickens, stirring frequently. Remove from heat. Stir in fontina, Swiss and Parmesan cheeses until melted. (Cheeses will be stringy.) Stir in wine.
Add cooked mostaccioli to cheese sauce; stir gently to coat. Pour into sprayed dish. Arrange sliced tomatoes around outside edge of dish. Brush tomatoes with oil; sprinkle with onions.
Bake at 350°F. for 20 to 25 minutes or until edges are bubbly and mixture is thoroughly heated.