Clean the carp from scales and gut the insides, making an incision along the abdomen. Wash the carcass inside and out. Peel the onion and cut into rings. Wash the lemon and cut into slices.
Mix salt and seasoning and rub the carp with this mixture. Then put it in the refrigerator for 30 minutes to marinate.
Place foil on a baking sheet, if it is thin, then in two layers. Grease with vegetable oil and place the carp on it. Mix mayonnaise with sour cream and brush the fish on top and inside. Using a sharp knife, make transverse cuts on the fish from head to tail. Insert lemon slices into the slits. Place the remaining lemon slices inside along with the onion.
Preheat the oven to 180 ℃. Fold the edges of the foil to cover the carp and place the baking sheet to bake for 40 minutes. Then remove, open the foil and leave for another 10 minutes until the fish browns.
Serve the finished fish on a platter, garnish with tomato slices, mint and parsley sprigs.