Sprinkle the bass cavity with salt. Wrap fish in cheese cloth and place in a deep baking dish. Boil 2 to 3 cups of water. Pour hot water into baking dish 3/4 inch deep. Cover dish with foil. Bake in oven at 400 degrees, approximately 10 minutes per inch of thickness or until the fish is close to the point of flakiness, but is still firm and moist. While fish is baking, in a medium bowl mix together sour cream, mustard and thyme. When cooked, remove fish from pan. Unwrap cheese cloth. Serve hot fish with cool mustard thyme sauce.