In large skillet, melt butter over medium-high heat; cook mushrooms, celery, onion, salt and pepper, stirring, for about 5 min or until tender but not brown. Stir in stock; bring to boil.
Stir in 1-1/2 cups (375 mL) of milk; bring just to simmer. Stir in chicken; reduce heat to low and barely simmer, stirring often, for 5 to 10 min or just until chicken is no longer pink inside. Push chicken and vegetables to one side of skillet.
Whisk flour into remaining milk; whisk into sauce side of skillet. Stir to combine well. Increase heat to medium and bring just to a simmer, stirring often. Cook, stirring often, for about 5 min or until sauce is slightly thickened.
Stir in asparagus and red pepper; simmer, stirring often, for about 5 min or until vegetables are tender-crisp.
