Marinate for 6-8 hours.
Pat duck breasts dry with paper towels, then remove excess fat. Using a knife, score the skin of the breasts in a crosshatch, making sure not to cut into the flesh.
Combine marinade ingredients in a bowl.
Transfer two thirds of the marinade to a larger bowl. Set the remaining marinade aside in the fridge. Add the duck breasts to the large bowl and toss to coat with marinade. Cover and let marinate for 6 to 8 hours in the fridge.
When ready to cook, preheat barbecue to medium high (about 220 °C – 425 °F).
Drain duck breasts and discard marinade.
Place duck breasts skin side down on the hot and oiled barbecue grill. Cook for 2 to 3 minutes. Turn over and continue cooking for 2 minutes.
Turn off the burners on one side of the barbecue. Place the breasts skin side down on the grill over the burners that are off for indirect cooking. Close the lid and cook for 10 to 12 minutes, until the internal temperature of the breasts reaches 137 °F (58 °C) on a thermometer. The breasts should be moderately firm to the touch and still pink inside.
Place the breasts on a plate and cover loosely with aluminum foil. Let rest for 5 to 10 minutes before cutting.
Meanwhile, mix the reserved marinade with the mango, cashews, red onion and cilantro in a salad bowl.
Serve breasts with Thai-style salad.
