1. Cook noodles according to package directions, drain and set aside.
2. In the meantime, slice mushrooms 1/4- to 1/3-inch thick. If large, cut in half. Sauté in oil over medium heat, until mushrooms are softened and darkened. Set aside.
3. Melt butter over medium heat. Mix with flour until well incorporated.
4. Slowly add milk, a little bit at a time, mixing well so no clumps form. Once you’ve added all the milk, sauce should be nice and creamy. Mix in salt, pepper and dry mustard.
5. Finally add in cheeses. Stir until melted and remove from heat.
6. Mix fettuccine and mushrooms into sauce. Garnish with basil.