Lightly season the chicken with salt and pepper. Heat 1 tablespoon oil in a large sauté pan or skillet over medium-high flame. Briefly sauté the chicken, stirring frequently. You don’t want to brown it, just cook off the pinkness (and honestly, flavor the pan with chicken). Chicken won’t be cooked through and that’s okay. Transfer to a bowl and reserve.
Reduce heat to medium and add onion, drizzling in more oil as needed. Cook, stirring frequently, until onion is translucent, 3 minutes or so.
Add garlic, ginger, curry powder and cumin, stirring to combine with onion. Cook for about 1 minute, stirring frequently and, again, drizzling in more oil if needed.
Transfer to a blender. Add coconut milk, 2 cups of the mango and 1 teaspoon each salt and pepper. Blend until smooth, creating the mango curry sauce.
Return sauce to skillet. Add potatoes and bring to a boil. Reduce heat to a simmer and cook, covered for about 15 minutes. If sauce gets too thick (which it may do on the bottom), stir in 1/2 cup water.
Stir in fish sauce and return chicken to pan. Cook for another 5 minute or so, until chicken is cooked through, adding a little more water if sauce gets too thick.
Stir in remaining cup of mango and remove from heat. Spoon alongside rice and serve.