1 – In a 1 1/2 quart saucepan, melt 1 tbsp butter over low heat. When butter begins to froth, add the flour. Mix well with a wire whisk and cook over medium heat, stirring constantly, until lightly golden. Add chicken broth, 1/2 tsp salt, 1/2 tsp pepper and nutmeg. Turn heat to low and cook, whisking constantly, until slightly thickened, about 2 minutes. Cover saucepan and set white sauce aside.

2 – In a large saute pan, heat olive oil over medium heat until haze forms, then add 2 tbsp butter. Add scallions and cook, stirring constantly, until barely tender, about 1 minute. Stir in prosciutto and cook, stirring, until softened but not brown, about 1 minute. Add mushrooms, turn heat to medium-high and cook, stirring, until mushrroms just begin to exude their juices, about 1 minute. Season with salt and pepper. Pour white sauce over mushroom mixture and blend well. Turn heat to low and cook, stirring, until mixture is well incorporated, about 2 minutes. Remove from heat.

3 – Cook pasta in 6 quarts boiling water with 1 tbsp salt until al dente. Drain in a colander, transfer to a bowl containing 1 tbsp softened butter and toss quickly. Mix 3/4 of the sauce and Parmigiano cheese with pasta. Spoon remaining sauce over pasta and garnish with minced parsley. serve with additional freshly grated Parmigiano cheese.