Preheat oven to 350 degrees F. Lightly mist a baking dish with non-stick vegetable oil spray.
Melt 1 tablespoon butter in a large skillet over medium high heat; add mushrooms and saute until they are golden brown. Transfer to a plate.
In same skillet, melt remaining 4 tablespoons butter; add onions and cook until they are soft and golden brown. Add flour and stir for 1 minute; pour in chicken broth and heavy cream and whisk until sauce is smooth. Mix in mustard, salt, pepper, thyme and parsley. Pour sauce into large mixing bowl.
Fold in chicken, carrots, peas, beans and mushrooms into sauce. Pour all into prepared baking dish.
Arrange sliced rolls on top of filling and brush with melted butter.
Bake 20 to 25 minutes or until filling is bubbling. If topping starts to brown too much, cover loosely with aluminum foil.