First, get a large pot of water on the stove for the pasta so that it is boiling by the time you need it.
Then get out a large skillet with deep sides and heat the Olive Oil (1 dash) in it over medium high heat. Brown the Boneless Skinless Chicken (1 pound) in it until completely cooked through for about 4-5 minutes. Season it with Salt (to taste) while it cooks. Transfer to a plate and set aside.
In the same pan, add the Cremini Mushroom (8 ounce) in. Let them start to soften for a couple of minutes before you add the Garlic (3 clove) and Butter (6 tablespoon). Let the butter melt and coat the mushrooms and garlic, then add the All-Purpose Flour (6 tablespoon). Stir it all well until the mushrooms become thick with roux covering it.
Slowly pour in the Milk (3 cup) while you continue to stir until everything is smooth. Then stir in the Dijon Mustard (1 tablespoon), Parmesan Cheese (1/4 cup) and another pinch of salt. Let it all cook and thicken for 6-8 minutes, stirring fairly often.
While the sauce cooks, cook the Rigatoni (1 pound) in the water you got boiling. It should get tender in 8 minutes or so. Reserve a scant 1/4 cup of the cooking water for the starchy goodness and add it to the sauce.
Then drain the pasta and add it to the pan of sauce, along with the chicken. Let everything gently simmer together for another couple of minutes while you stir it together thoroughly.
Scoop into pretty bowls and serve immediately with extra parmesan on top! It also reheats really well for fantastic leftovers.