Preheat the oven to 375 degrees F (190 degrees C).
Grease a 9 or 10 inch (23 to 25cm) square baking dish with cooking spray.
Put the cottage cheese, mustard, skim milk or buttermilk, nutmeg, cayenne, salt and black pepper in a blender.
Puree until smooth then pour into a bowl.
Stir in the onions, uncooked macaroni and cheddar cheese.
Transfer the mixture into the baking dish.
Toss the parmesan with the breadcrumbs and sprinkle over the top.
Bake for 45 minute or until the center is firm and the topping is golden brown.