Preheat the oven to 190C/Gas 5. Coat a 20 by 33cm baking pan with cooking spray.
Bring a large pot of water to a boil. Add the macaroni and cook until tender but firm, about 5 to 8 minutes. Drain and transfer to a large bowl.
Meanwhile, place the frozen squash and milk into a large saucepan and cook over a low heat, stirring occasionally and breaking up the squash with a spoon until it is defrosted. Turn the heat up to medium and cook until the mixture is almost simmering, stirring occasionally. Remove the pan from heat and stir in the Cheddar, jack cheese, ricotta cheese, salt, mustard and cayenne pepper. Pour cheese mixture over the macaroni and stir to combine. Transfer the macaroni and cheese to the baking dish.
Combine bread crumbs, Parmesan cheese and oil in a small bowl. Sprinkle over the top of the macaroni and cheese. Bake for 20 minutes, then grill for 3 minutes so the top is crisp and nicely browned.