- Preheat oven to 350°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 sauté 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.