Freeze two heads of cabbage for about 3 days. Transfer to the refrigerator to defrost the day before assembly.

Bring a large pot of water to a boil, add rice, and cook until al dente (about 5 minutes). Drain and set aside to cool.

Heat olive oil in a large skillet over medium heat. Sauté chopped onion for 7-10 minutes until soft. Add grated carrots and cook for an additional 2-3 minutes.

In a large mixing bowl, combine sautéed onion and carrots with ground beef, ground pork, parsley, cooked rice, salt, and pepper.

Whisk together chicken stock, crushed tomatoes, and sour cream in a separate bowl. Season with salt and pepper.

Cut away the thick rib from each cabbage leaf. Spoon 2 tablespoons of filling onto each leaf, fold in the sides, and roll tightly.

Pour a cup of sauce into a casserole dish, arrange cabbage rolls seam-side down, and layer with bay leaves and more sauce.

Cover with a lid or foil and bake in a preheated oven at 350°F (180°C) for 1 hour, then uncover and bake for an additional 30 minutes.

Remove from the oven and let the rolls rest for 10 minutes before serving. Optionally serve with more sour cream or crème fraîche and fresh herbs.