• Preheat the oven to 350. 

Mushrooms

• Heat the butter and oil in a small skillet.
• Add the sliced mushrooms and cook about 5 minutes, salting to taste. Remove to a bowl and set aside.

White Sauce and Pasta

• Bring a pot of water to boil. Break the linguini noodles in half and cook them while you make the sauce. 
• In the same skillet you cooked the mushrooms in, heat the butter until melted, then stir in the flour. Use a whisk or a flat bottomed wooden utensil to incorporate the flour and butter, then cook, stirring, another minute or two, until it's light brown. 
• Slowly whisk in the chicken broth, making sure to eliminate any lumps. Bring to a simmer and whisk in the milk, then add the nutmeg. Cook about 5 minutes, stirring frequently, until the sauce thickens.
• Take the pan off the heat and stir in the grated cheese, mushrooms, and chicken. As soon as the pasta is done, drain it and stir it into the sauce.
• Butter a small baking dish and pour the tetrazzini in. 
• Stir together the bread crumbs and the rest of the cheese and sprinkle over the sauce.
• Bake at 350 for 30 minutes.