Cut the white skin of the duck breast into a diamond shape. Season with salt and pepper on both sides. Heat a pan without fat over medium heat. Place the duck breast skin side down in the pan, fry over low to medium heat for about 15 minutes. Turn the meat over and fry for another 5 minutes.
Mix the vinegar, oil and 1 tablespoon of the duck fat that has escaped. Season the marinade with salt, pepper and sugar. Halve the cherry tomatoes, slice the parmesan cheese into fine slices using a peeler.
Cut the still warm duck breast diagonally into thin slices. Arrange the rocket and tomatoes on plates and serve the meat on top. Sprinkle with parmesan. Drizzle the marinade over it just before serving.