Bring 1 litre water to a boil in med. saucepan over high heat. Add potatoes, 1 cup celery and quarter cup celery leaves. Reduce heat to med.; simmer, covered 7-8 min. or until potatoes are tender. Drain, reserving cooking liquid and vegetables separately. Heat oil in large nonstick skillet over med.-high heat. Add chicken, bell pepper and remaining 1 cup celery. Cook, stirring constantly, 6-7 min., or until chicken is no longer pink. Stir in tomato paste, rosemary, sugar, black pepper and 2 cups reserved celery liquid. Bring to a boil; cook until sauce thickens, about 4 min. Add reserved potatoes and celery. Season to taste with salt. Sprinkle with remaining celery leaves.