Melt butter in 1 quart saucepan; add onion and celery and saute 5 minutes. Blend in flour. Remove from heat; gradually add milk. Cook over medium heat, stirring constantly, until thickened. Cook 2 additional minutes. Add Cheddar cheese and stir until melted. Stir in stock and pepper. Heat to serving temperature. Do not boil. Serve garnished with parsley.