Score the fat on the duck breast in a cross-hatch pattern but be careful not to cut the meat. Place the duck breasts in a pan with the fatty side downwards. Cook this side until golden brown and then turn.
When the other side has a nice colour, you can place the duck breasts in the oven until the core temperature is 50 degrees. Let the meat rest for 15 minutes under aluminium foil. Cut each breast into 2 portions before serving.
Wash and peel the sweet potatoes, and cut them into cubes. Boil the sweet potatoes in a pan with water and salt until done. Drain the sweet potatoes and let them cool slightly. Mash the sweet potatoes and then press them through a strainer. Add salt to taste.
Mash the boiled potatoes. Slowly add the flour and finally the egg yolk. Knead into a nice dough. Put the dough in a sealable zip bag and roll the dough in the bag into a flat dough of 2 cm thick. Seal the zip bag tightly and boil in a pan with plenty of water until the gnocchi is ready (roughly 7 minutes). Place the gnocchi in the refrigerator to cool and cut into rectangles. Fry these in the fryer until golden brown and sprinkle them with sea salt when done.
Peel the pearl onions. Put the butter, white wine, the bay leaf and the sprigs of thyme in a pan. Fry the onions in this mixture on low heat. When the onions are done, pat them dry and torch them with a crème brûlée torch.
Place a pan on the stove with the stock and reduce it to a third. Add 200 grams of fresh blackberries and puree the gravy. Save the rest of the blackberries to garnish the plate later on.
Make a quenelle from the sweet potatoes and place it on the plate. Lay the gnocchi against it and the duck breast on the other side. Garnish each plate with 2 torched onions and some blackberries. Pour out the gravy when the dish is served.