Peel and finely chop the ginger. Simmer mirin, soy sauce and sugar for 5 minutes. Season with cayenne pepper. Let it cool down. Rub the marinade into the duck breasts. Leave covered to marinate for 1-2 hours.
Heat the oil in a pan. Dab duck breasts slightly dry. Sear duck breasts briefly on both sides. Place in a fireproof dish with the skin side facing upwards. Set aside the pan for the blueberry sauce.
Roast in the preheated oven at 200 degrees (gas: level 3-4, fan oven: 180 degrees) for about 15 minutes.
Add blueberries to the pan and defrost while stirring. Add the rest of the marinade and duck stock and bring to the boil. Whisk the starch with a little cold water. Stir into the sauce and bring to the boil once. Season with 1 pinch of salt.
Let the duck breast rest for 5 minutes before cutting it. Rinse Thai asparagus, cut off the ends and cook in salted water for 3-5 minutes. Drain the asparagus. Cut the duck breast open and serve with the blueberry sauce and the asparagus.
Serve with basmati rice as desired.