Mix the chicken breast in a bowl with the mustard, salt, pepper and the herb mix. Preheat oven to 180 degrees.
Fry the leek and chicken breast in a pan with a little oil. Bring a pot of salted water to the boil in between. As soon as the meat and leek mixture is well seared, pour it into a baking dish.
Pour the gnocchi in portions into the sparkling salt water and as soon as they float on the surface, remove them, drain them and add them to the baking dish. Mix everything well at the end.
Mix the sour cream with the pesto and pour over the mixture in the baking dish. Sprinkle with cheese and put in the oven for about 40 minutes.