Preheat the oven to 350 degrees F.

To make the sauce: melt the butter in a saucepan over medium-high heat. Add the onions and cook, stirring, for 3 minutes. Add the garlic and cook, stirring, for 1 minute. Add the tomatoes and cream and simmer, stirring occasionally, for 5 minutes. Add the vinegar and sugar and simmer, stirring occasionally, until the sauce thickens, about 5 minutes. Remove from the heat and adjust the seasoning to taste.

Separate the cabbage leaves and remove the hard spine from each leaf. Spread on paper towels and pat dry. Set aside.

In a medium skillet, melt the butter over medium-high heat. Add the onion and cook, stirring, until softened and starting to caramelize, about 5 minutes. Add the garlic, and cook, stirring, for 1 minute. Remove from the heat and let cool slightly.

Lay the cabbage leaves, rib side down, on a flat work surface. Spread a thin layer of the sauce over the bottom of a baking dish.

To make the filling: in a large bowl, combine the beef, pork, rice, Essence, salt, pepper, egg, and cooked onions. Mix well with a heavy wooden spoon or your hands.

One at a time, spoon the filling into the center of the cabbage leaves, about 1/4 cup in each, depending upon the size of the leaves. Roll each into a neat cylinder and place in a layer on the sauce in the baking dish. Repeat with the remaining ingredients, stacking the cabbage packages, as necessary. Pour the remaining sauce over the rolls, cover tightly with aluminum foil, and bake until the meat is cooked through and the rolls are tender, about 2 hours. Remove from the oven and serve the rolls with the sauce spooned over the top.