Place chicken breast in a pot with leek, carrot, celery, both halves of the small onion and the curry leaves, if using. Add enough water to just cover chicken and add salt. Bring to a low, rolling boil and reduce heat to keep at rolling simmer; cook 30 minutes, turning once then let chicken cool in cooking liquids. Remove chicken from skin and bones and pull into bite-sized pieces or dice; strain liquids. You’ll have more than you need for the recipe so freeze or refrigerate extra stock to use later.
Heat a deep skillet over medium heat, add butter and melt. Add large chopped onion, apples, ginger, garlic and season with salt and pepper. Cook 10-12 minutes to soften then add curry powder; stir a minute, add flour and stir a minute more. Add 3 cups stock and let thicken a bit, add chicken and 1/4 cup chutney. Simmer a few minutes to combine flavors then taste to adjust seasoning