Cut meat into 2x2 cm pieces. From meat scraps and bones, prepare 500 ml (about 2 cups) meat broth by simmering for 20-30 minutes.
Clean forest mushrooms and boil for 8-10 minutes. Drain well, then fry with finely chopped onion, salt, and pepper in a pan over medium heat for 5 minutes. Grind the cooled mushroom mixture in a meat grinder to make a paste.
Rinse buckwheat cereal and cook in water for 15-20 minutes until tender. (Optional: you can use raw buckwheat without cooking.)
Cut out the core from the cabbage head. Blanch the whole head in boiling salted water for 8 minutes. Carefully remove the softened leaves, trim away thick ribs, and reserve some torn leaves for the baking dish.
Mix the cooked buckwheat with the ground mushroom paste in a bowl.
Season the meat pieces with salt and pepper. Heat 2 tbsp fat in a large pan over medium-high heat and brown the meat for 5-8 minutes. Add finely chopped onion and cook for 3-5 minutes. Sprinkle 1 tbsp flour over the mixture, stir well, and cook for 1 minute.
Reduce heat to medium. Stir in 150g sour cream and 1 tsp dry marjoram. Add more spices if desired. If the sauce is too thick, thin it with a little broth. Keep warm.
Place a full tablespoon of buckwheat-mushroom filling on each cabbage leaf. Fold in the sides and roll tightly to form a compact roll.
Line a baking dish with torn cabbage leaves. Arrange the rolls seam-side down and pour salted broth to come halfway up the rolls. Cover the top with more cabbage leaves.
Bake at 200°C (392°F) for 40 minutes.
Remove from oven and carefully discard the top cabbage leaves. Distribute the meat sauce evenly over the rolls. Return to the oven at 180°C (356°F) and bake for 30-40 minutes until the pork reaches an internal temperature of 63°C (145°F).
Garnish with chopped fresh parsley and dill. Serve immediately.