Place chicken in a bowl and toss with cornstarch. Place oil in a large, wide skillet set over medium-high heat. When hot, add chicken and brown well. Add onion, celery and ginger and cook 2 minutes. Mix in curry powder and cook 1 minute. Add the coconut milk, stock, lime juice and sugar. Bring to a slow simmer, adjusting the heat downward to maintain that simmer. Simmer 5 minutes. Add apples and simmer 10 minutes more. Mix in the mint or cilantro, season with salt, and then serve.