Heat oil over medium-high in a large skillet with high sides or Dutch oven. Stir in onions and celery, and sauté until softened, about 7 minutes.
Stir in apples and garlic, and cook for 3 minutes. Push everything to the outside of the skillet.
Put chicken in the middle of the skillet. Sprinkle everything with curry powder and salt. Cook for 3 minutes, then stir.
Pour in 1 ¾ cup chicken broth, and scrape up any bits that have gotten stuck to the bottom of the skillet. Turn heat down to medium, and simmer for at least 5 minutes, stirring occasionally.
About 5 minutes before ready to serve, whisk together cornstarch and the remaining ¼ cup broth until no lumps remain. Pour into skillet, stir, and simmer until the sauce has thickened, about 5-10 minutes.
*Makes about 6-7 cups Chicken Apple Curry sauce.*Recipe tested using Fuji apples and Honeycrisp apples.*Use cold broth or water when whisking with cornstarch.*Toppings to add: plain yogurt, fresh mint or parsley, shredded unsweetened coconut, chopped peanuts, cashews or almonds, and chopped green onion