Dice the chicken breast and fry in butter for 5 minutes. Chop the leek, tomatoes and peppers and add to the pan. After 3 minutes, deglaze with the broth and simmer for 5 minutes. Add the lentils and let simmer for another 10 minutes. Stir in the cream and mustard, season with the spices and lemon juice and simmer for 5 minutes.