Line the baking dish with a 13x18-inch Classic Ovenable Pan Liner. Place the seam in the center and over the pan. Crimp the edges of the liner around the rim of casserole dish; set aside.
Place butter, onion and garlic in large saucepan. Cook over medium heat until onion is tender. Add flour and whisk until smooth. Pour in milk and chicken stock, whisking constantly. Heat until mixture is thickened and smooth.
Add three cheeses; stir until cheese is melted. Stir in cooked macaroni.
Pour mac and cheese into Ovenable Pan Liner casserole dish.
Bake uncovered for 35 to 45 minutes or until golden brown and bubbly.
