Peel and roughly chop the onion. Clean, wash and drain the leek and carrots. Cut carrots with a julienne slicer, leek with a knife into long, thin strips.
Wash the duck breast, dab dry well and make a diamond-shaped incision in the skin. Heat a flat roasting pan or an ovenproof pan without fat and place the duck breasts with the skin side down.
Fry vigorously for approx. 5 minutes until the skin side is golden brown. Turn the duck breast and fry on the meat side for 2-3 minutes. Season with salt and pepper. Add the onion and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes.
Roast the sesame seeds in a pan without fat and take them out immediately. Heat 20 g fat in a hot pan and bake the frozen croquettes in it, turning them according to the instructions on the packet until golden brown. Heat 30-40 g fat in a second pan and steam the vegetables for approx. 5 minutes, turning them over.
Season with salt and pepper. Remove duck breast from the roaster and let it rest for about 5 minutes. Drain the duck fat, remove the onion and dissolve the roast with broth. Add balsamic vinegar, bring to the boil and let it boil down for 1-2 minutes.
Stir in the sauce thickener, bring to the boil again briefly and season to taste with salt, pepper and possibly a little more vinegar. Cut the duck breast into slices and arrange on a plate with the vegetables. Spread balsamic sauce over it and sprinkle with sesame seeds.
Serve garnished with basil. Serve croquettes separately.