1. Preheat oven to 425°.
2. Combine squash, 1 1/2 teaspoons oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper on a jelly-roll pan coated with cooking spray. Bake at 425° for 24 minutes or until tender, stirring once.
3. Cook the pasta according to package directions, omitting salt and fat. Drain pasta; keep warm.
4. Heat a medium saucepan over medium-high heat. Add remaining 1 teaspoon oil to pan; swirl to coat. Add onion and garlic; sauté 3 minutes. Reduce heat to medium-low. Stir in 1/2 cup milk, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper. Combine remaining 1/2 cup milk, flour, and yolks in a small bowl, stirring with a whisk. Slowly add egg mixture to pan, stirring constantly with a whisk. Cook 5 minutes or until thickened, stirring frequently with a whisk.
5. Combine squash, pasta, and milk mixture in a large bowl; toss gently to coat. Sprinkle with cheese and nuts.