Finely shred peel from lemon; set peel aside. Juice lemon and set juice aside. Season chicken lightly with salt and pepper.
In a 4-quart Dutch oven heat oil over medium-high heat; add chicken. Cook for 2 to 3 minutes or until chicken is brown, turning occasionally.
Add carrots, gravy, and the water to Dutch oven. Stir in mustard. Bring to boiling. Place bok choy on top. Reduce heat. Cover and simmer about 10 minutes or until chicken is no longer pink (180 degrees F) and vegetables are just tender. Add lemon juice to taste.
Ladle soup into bowls. Sprinkle with lemon peel and lemon thyme.