Smothered Chicken

1 lb boneless chicken breasts
2 Tbsp butter
1 C chopped carrots
1 C chopped celery
1/2 C chopped onion
1/8 C flour
8 oz sliced fresh mushrooms
1 can chicken broth
1/4 tsp dried thyme

Salt and pepper chicken. Brown chicken in butter. Remove chicken from pan and add onion, carrots and celery. Cook 5 min. Add flour and cook 2 min. Add broth and when thick, add thyme and salt and pepper to taste. Put chicken back in and add mushrooms. Serve over Rice Pilaf.