Slice white and light green parts of 1 of the leeks into 1-inch (2.5 cm) pieces; place in large saucepan. Add chicken, stock, onion, bay leaf, thyme and peppercorns; bring to boil. Reduce heat, cover and simmer for 30 to 40 minutes. Remove chicken pieces. Strain liquid and discard vegetables and seasonings. Return broth to pot.

Slice white part only of remaining leeks; add to broth along with carrots. Return to boil; reduce heat, cover and simmer for 15 to 20 minutes or until carrots are tender. De-bone chicken thighs and cut into bite-size pieces. Add to soup and heat through. Season to taste with salt and pepper.