To prepare the sauce, place the walnuts, bread, yoghurt, vinegar, garlic in a blender, season with salt, pepper and lemon juice, then beat well until you get a homogeneous mixture. For the filling, finely chop the olives, dill, parsley, add the olive oil, pepper, salt, a little lemon juice and mix well.
Fill the fish with the stuffing, then close them with toothpicks to make sure the filling does not spill out of the fish.
Leave the stuffed shads overnight in the refrigerator, covered with foil. The next day, bake the fish on the grill, and if desired, fry them.
Serve with a slice of lemon and green olives, with the sauce in a separate pitcher.