Melt butter in large skillet over medium low heat.
Add the onion, apple and garlic and mix well.
Cook, covered over low heat until the onion is tender and translucent, about 12 minutes.
Add the curry powder to the onion mixture and mix well.
Dissolve cornstarch in the milk and add it to the mixture.
Add the chicken broth and mix well.
Reduce the heat to low and cook, stirring frequently, until slightly thickened.
Add the lemon zest and lemon juice and mix well.
Stir in the chicken and continue cooking until the chicken is heated through.
Optional: Serve over rice with condiments such as chutney, raisins, diced bell peppers, cucumber, tomato, pineapple, papaya, mango, banana, kiwi, chopped scallions, hard-boiled eggs, peanuts or toasted almonds.