Preheat oven to 350°F, placing a rack in the center position.
Wash the kale and remove the thick spines down the center. You should be able to peel the dark, leafy portion right off. Discard spines, placing the leaves in the bottom of a deep casserole dish.
Place the whole roaster chicken, breast side up, on top of the kale.
Gently lift the edge of the skin at the neck and rub butter beneath the skin, pushing it in as far as you can get it. Rub the remaining butter all over the surface of the chicken skin.
Rub the outside of the chicken evenly with salt, pepper, and dried rosemary.
Place the lid snugly on the casserole dish and bake for 45-55 minutes or until juices run clear.
Remove the lid. Turn the heat up to 425°F and continue baking another 15 minutes or until the chicken skin is crisp and golden.
Start preparing the mushrooms as soon as you remove the casserole lid and return the chicken to the oven.