Start by making the marinade. Mix all the ingredients. Cover the duck breast with the marinade in a bowl and refrigerate for 24 hours. Heat the mixture to 160 degrees (73 C) and let it cool off. 

Add the vegetables, apples, mustard and butter to a pan and over medium heat.
Turn up the heat and let it cook until the apples are staring to get soft, but not soft all the way through. It will take about 7-8 minutes.

After the heating in the marinade, the duck breasts are already starting to warm inside. Season them with salt and sear them off in a pan over high heat -- skin side down (you will not need any butter) for 6-7 minutes, until the internal temperature is 145 degrees. Turn off the heat when the breasts have got a brown, crisp surface and let them rest in the pan for a few minutes. Slice the breasts into very thin slices and add the apple glace. Sprinkle with salt.