Bring a large pot of water to a boil. Cook tortellini (or rigatoni) until floating.
Drain and set aside.
Heat your oil in a large non-stick skillet over medium heat. Brown your chicken 1-2 minutes.
Add your mushrooms and cook until chicken is done. (If using fresh mushrooms, cook until they are golden brown.)
Mix in your sour cream and cook until mixture is creamy and slightly thick.
Spray a 9x13 pan with cooking spray. Add tortellini (or rigatoni) to the mixture and pour into pan.
Add cheese on top of mixture so that it completely covers.
Cover with foil and bake at 350 degrees for 30 minutes.
Uncover and bake for an additional 15 minutes, until bubbly and golden brown around edges.