Trim the base of the cabbage to loosen the leaves, then carefully separate them. Bring a large pot of water to a gentle boil and blanch the leaves for 5-7 minutes for regular cabbage and 4-5 for Napa, until the stems are pliable but not mushy and still hold their shape. Timing may vary depending on your heat, so keep an eye on them.
Drain the leaves and spread them out on a clean kitchen towel or a wire rack-lined sheet pan to dry off excess moisture.
Melt the butter in a skillet over medium heat. Add the diced onion and sauté for 5-7 minutes, stirring occasionally, until soft and translucent. Stir in the grated carrots and minced garlic, cooking for another 3-5 minutes until soft and fragrant. Remove from heat and let cool.
In a large mixing bowl, combine the ground beef, ground pork, parsley (if using), sautéed onion mixture, salt, and pepper. Blend everything evenly, ensuring the flavors meld together.
In a medium bowl, whisk together the water, sour cream, tomato paste, and a pinch of salt and pepper until smooth. Taste and adjust seasoning as needed. Set this creamy mixture aside.
Preheat your oven to 350°F. Take a cabbage leaf and trim down the thick central rib with a knife for easier rolling. Place about 2 tablespoons of the meat filling near the wide base of the leaf. Roll it up like a burrito—fold the bottom over the filling, tuck in the sides, and roll tightly to the top. Sizes may vary slightly depending on your cabbage leaves.
Arrange the rolls seam-side down in a deep baking dish or Dutch oven, stacking them snugly. Pour the prepared sauce evenly over the top, nestle the bay leaves among the rolls, and cover with a lid or tightly sealed foil. Bake for 45 minutes for green or white cabbage and 35-40 minutes for Napa, then uncover and bake for an additional 30 minutes to let the tops develop a golden, slightly caramelized finish.
Let the rolls rest for 5-10 minutes after baking, then serve warm with a dollop of extra sour cream and a sprinkle of fresh dill or parsley for a bright, herbaceous touch.