Preheat oven to 375° F.

Cook the macaroni in boiling, salted, water until tender.
Drain and keep hot.
In a mixing bowl, combine 1 cup of the cheese with the flour, onion, mustard, salt, and pepper.
Add to the hot macaroni and mix well.
Transfer to a buttered 2 ½ quart casserole dish.
Pour the milk over the macaroni.
Top with the remaining cheese and dot with butter.

Bake for 45 minutes, or until the macaroni is firm and the top is golden brown.