Cut shallot in small cubes and saute in sunflower oil. Add the lentils, stir, pour the broth, cook for 40 minutes over low heat. Incise the skin of duck breast diamond-shaped, rub with salt and pepper. Fry for 4 minutes on the skin, drain excess fat, turn and cook for another 8 minutes. Red onion cut into strips and mix with vinegar, salt, pepper, mustard, sugar and olive oil. Drain lentils. Duck cut into slices. Chop salad. Cherry tomatoes cut in half. Mix the lentils with duck, put on a green salad, pour the sauce and garnish with tomatoes.