1. Preheat oven to 425°. Melt butter in oven in a lightly greased 13- x 9-inch baking dish (about 3 minutes).
2. Meanwhile, dip chicken in 1/2 cup buttermilk; sprinkle with pepper, and dredge in flour.
3. Remove baking dish from oven. Add mushrooms and thyme. Arrange chicken, breast sides down, in baking dish.
4. Bake at 425° for 25 minutes. Turn chicken, and bake 10 minutes.
5. Stir together soup and remaining 1 cup buttermilk. Pour over chicken, and bake 10 minutes or until a meat thermometer inserted into thickest portion registers 165°, shielding chicken with aluminum foil to prevent excessive browning, if necessary.
6. Transfer chicken to a serving platter. Whisk soup mixture in baking dish until blended, and drizzle over chicken.