Wash hens and pat dry. Make four 1/2 inch deep slashes in breasts. Place 4 mushrooms and a lemon slice in each cavity. Tie legs together. Place on rack of a steamer. Stir together hoisin, water, five spice, and lemon peel. Brush 2/3 of mixture over hens, pressing into slashes. Cover steamer and steam for 50 minutes. Add pepper and bok choy. Steam 10 minutes until hens and vegetables are tender. Brush hens with remaining sauce before serving