Preheat oven to 350

Cook noodles according to package directions, drain, and transfer to a large bowl. Gently fold in tuna, mushrooms and beans. (if you saute the mushrooms beforehand it will cut down on the moisture in the casserole)

Whisk sour cream, mayo, mustard, parsley, salt & pepper together in a separate bowl; fold into the noodle mixture.

Spread mixture into a greased 4-quart casserole dish, and top with cheese. Bake until top is golden and bubbly, about 30 minutes.