In a 4-quart Dutch oven heat oil over medium heat. Add chicken, carrots, celery, leeks, and garlic. Cook and stir for 5 to 8 minutes or until chicken is brown on all sides and vegetables are starting to soften. Stir in chicken broth, potato, green beans, rosemary, and ground black pepper. Bring to boiling; reduce heat. Simmer, covered, for 20 to 25 minutes or until vegetables are tender and chicken is no longer pink.
Meanwhile, in a small bowl whisk together milk and flour until smooth. Stir mixture into cooked stew mixture. Return to boiling; reduce heat. Cook and stir about 2 minutes or until mixture is thickened. To serve, ladle stew into four bowls. Sprinkle each serving with cracked black pepper.