Halve the carrots lengthways and cut into pieces. Cut the leek into rings, dice the onion. Wash the chicken breast, pat dry and cut into bite-sized pieces.
Heat the oil in a pan and fry the chicken cubes all around. Add the carrots, leek and onion and sauté. Season with salt and pepper. Deglaze with broth and cook for 8 minutes over low heat. Add the cream and mustard and bring to the boil briefly, season again with salt and pepper.