1.In a saucepan of boiling, salted water, cook macaroni according to packet directions. Drain.
2.In a saucepan, melt 60 grams of butter on medium heat. Stir in flour and cook, stirring, 1 minutes. Stir in milk, increase heat and stir until sauce boils and thickens. Stir in 1/2 cup of cheese and asparagus. Add nutmeg and season to taste.
3.Toss through pasta, tuna and remaining butter. Pour into a 6-cup gratin dish. Sprinkle with remaining cheese and place under a preheated grill 5 minutess, until cheese melts and browns. Serve with salad.