Cook cabbage leaves in boiling water to cover for 6 minutes. Drain and pat dry. Mix together the chicken, onion, celery, mushrooms, garlic, salt and thyme; stir in egg. Place 1/2 cup of the chicken mixture in the center of each cabbage leaf. Fold sides of the cabbage leaf towards center, over filling, and then fold and overlap ends to make a small bundle. Fasten with wooden picks. Place in 13 x 9 x 2-inch baking dish. To make sauce, melt the butter in a large skillet. Blend in flour; blend in sour cream and paprika and then gradually stir in chicken broth. Cook over low heat, stirring constantly, until thickened. Pour sauce over cabbage rolls. Cover baking dish and bake in 350 F oven for 35 minutes. Remove picks to serve. Makes 4 servings, 2 cabbage rolls each.