Place peas and carrots in saucepan with a small amount of salted water. Simmer, covered, 10 minutes, drain. In a large saucepan melt butter. Simmer onion and celery in butter until tender. Remove from heat. Blend in flour. Gradually stir in chicken broth and milk. Add thyme. Cook, stirring constantly, until mixture thickens and comes to a boil. Add chicken and drained peas and carrots; mix well. Turn into 4 quart casserole. Top with biscuits in a ring around the edge of casserole. Bake at 400 for 18 to 20 minutes, until biscuits are browned.