Prepare the cabbage: Bring a large pot of water to a boil. Cut the core out of the cabbage, then carefully peel off 12-15 large leaves. Boil the leaves for 2-3 minutes until softened. Drain and set aside to cool.
Cook the filling: In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Add the ground beef (or turkey) and cook until browned. Stir in the garlic powder, oregano, salt, and pepper. Remove from heat and mix in the cooked rice.
Assemble the rolls: Lay a cabbage leaf flat and place a couple of tablespoons of the meat mixture in the center. Fold in the sides of the cabbage leaf and roll tightly. Repeat with the remaining leaves and filling.
Prepare the sauce: In a separate bowl, combine the diced tomatoes, tomato sauce, brown sugar (if using), and lemon juice (if using). Pour a small amount of the sauce into the bottom of a large baking dish.
Bake the cabbage rolls: Place the rolled cabbage rolls in the baking dish, seam side down. Pour the remaining tomato sauce over the top. Cover with foil and bake at 350°F (175°C) for 45-60 minutes, until the cabbage is tender.
Serve: Garnish with fresh parsley before serving if desired. Enjoy!