Preheat oven to 325°F. Cook sausage, celery, onion, carrots, garlic and rosemary in large stockpot over MEDIUM-HIGH heat for 8 minutes. Add spinach and mushrooms; cook 2-3 minutes longer or until sausage is thoroughly cooked, stirring frequently. Remove from heat.
Add stuffing mix, chicken broth, black pepper and egg; mix just until blended.
Spoon into lightly greased 13 x 9-inch baking dish; cover with foil.
Bake 50 minutes to 1 hour or until thoroughly (160°F).