Preheat oven to 350 degrees.

Cook pasta according to the package directions, omitting salt and fat; drain.
Lightly coat pasta with cooking spray.

Heat a Dutch oven over medium-high heat. Coat pan with cooking spray.
Add onion and carrot, and sauté 4 minutes. Add garlic; sauté 1 minute.
Add ground sirloin; cook 5 minutes or until browned, stirring to crumble.
Add tomato sauce, 1/2 teaspoon salt and pepper.
Cook for 2 minutes or until most of liquid evaporates.

Add pasta to beef mixture in pan, stirring to combine.
Spoon pasta mixture into an 11x7-inch baking dish coated with cooking spray.

Place milk, flour, nutmeg, and remaining 1/2 teaspoon salt in a medium saucepan; stir with a whisk until blended. Cook over medium heat 2 minutes or until thickened, stirring constantly with a whisk. Add 1 cup cheese, stirring until smooth.
Pour cheese mixture over pasta mixture; stir.
Top evenly with remaining 1/2 cup cheese.

Bake at 350 degrees for 20 minutes or until lightly browned.
Let stand 5 minutes before serving.