Wash the duck carcass thoroughly and pour boiling water over it. After this, place on a paper towel and dry. Pour boiling water over the lemon. Soak the buckwheat for an hour, during which time change the water several times.
In a small bowl, combine marinade spices, salt, sunflower oil, bay leaf and lemon juice. Place the duck there and coat the marinade well on the outside and inside, rubbing it in thoroughly with your hands. Cover the duck with cling film and leave in a cool place for 3 hours.
Boil buckwheat in salted water until tender. After cooking, it should be dry and crumbly, not watery. Stuff the marinated duck with cooked buckwheat. Secure the duck skin around the cut with toothpicks.
Place the duck in the roasting sleeve and secure the edges of the sleeve with clamps. Place the sleeve on a baking sheet and place in the oven preheated to 180°C for 2 hours. Twenty minutes before the end of baking, cut the sleeve and remove it to allow the duck to brown.
Before serving, transfer the cooked duck to a platter and remove the toothpicks. Serve it hot as a meal on its own. When serving portioned, place a portion of meat and buckwheat on a plate, decorate the dish with fresh herbs and cherry tomatoes.