In a bowl, season with salt and pepper to taste. Add 1 tablespoon of the curry powder and massage onto chicken. Marinate for about 30 minutes.
In a wide pan over medium heat, heat oil. Add onions, garlic, and ginger and cook, stirring regularly, until softened.
Add chicken and cook until lightly browned.
Add fish sauce and cook for about 1 to 2 minutes.
Add coconut milk and curry powder. Stir to combine.
Lower heat, cover, and cook for about 10 to 15 minutes or until chicken is cooked through and sauce is reduced.
Add mango chunks, mashing with the back of the spoon. Continue to cook for about 2 to 3 minutes or until slightly thickened.
Season with salt and pepper to taste. Transfer to a serving platter and garnish with chopped cilantro, if desired. Serve hot.
Notes
The overall flavor of the dish depends on the fruit used. If the mangoes are fully ripe and very sweet, balance the dish with a tablespoon or so of lemon juice or vinegar. If the mangoes are sour, add honey or brown sugar to taste.