In large skillet, melt 1 tablespoon of the butter over medium heat. Add onion, celery and garlic. Cook 8 to 10 minutes or until tender, stirring occasionally. Add mushrooms. Cook over medium-high heat for 6 to 8 minutes or until mushrooms are tender and most of the liquid is evaporated, stirring occasionally. Add cream. Bring to boiling; reduce heat. Simmer, uncovered, for 3 minutes. Stir in rice, bread crumbs, 1/4 teaspoon salt and 1/8 teaspoon pepper. Cover and chill at least 2 hours before using.
Make a horizontal pocket in each chicken breast half by cutting from one side almost to, but not through, the other side. Spoon chilled stuffing into chicken pockets. Don't worry if the stuffing isn't totally enclosed by the chicken.
Line a 15x10x1-inch baking pan with foil; coat foil with nonstick cooking spray. Arrange stuffed chicken breast halves in foil-lined pan.
Melt the remaining 1 tablespoon butter and brush atop chicken breast halves. Season with additional salt and black pepper. Bake, uncovered, in a 375 degree F oven for 30 to 35 minutes or until internal temperature of chicken breast halves reaches 170 degrees F on an instant-read thermometer. Makes 6 servings.
Prepare as above through step 3. Cover and chill stuffed chicken breast halves up to 24 hours. Uncover and continue as directed in step 4.