Preheat oven to 375 degrees.
Bring large pot of water to boil and cook macaroni per package instructions. Drain and rinse with cold water.
Melt butter in large saucepan and sauté onion, garlic, and sage until onion is transparent. Stir in flour and cook over low heat for several minutes, until very thick. Gradually stir in milk and cook until slightly thickened, stirring constantly. Add all but 1/2 cup Cabot Cheddar and stir just until melted. Remove from heat and fold in turkey and apples.
Pour into 9"x13" greased pan. Top with crushed croutons and reserved Cabot Cheddar. Bake until lightly browned and bubbling - about 25-30 minutes.