Reduce heat to medium-low and add chopped onions to skillet. Cook for 10 minutes, until soft and golden. Add garlic and continue cooking for 2 minutes. Add cayenne and curry leaves, and saute for 1 minute. Return chicken to skillet, and add ketchup and chicken broth. Simmer uncovered for 20 minutes or more, until chicken is cooked and tender, and sauce has thickened.