Wash and core the cabbage. Cut into quarters and pull apart roughly into chunks about 4 leaves thick.
In a large bowl, mix the ground meat with the rice, paprika, fennel, ½ teaspoon thyme, ½ teaspoon salt, and allspice plus half of each of the garlic and onions. Mix gently; feel free to use your hands.
In a 4-cup or larger glass measuring cup, measure the tomato sauce and mix with the tomato paste, diced tomatoes, ½ tsp. salt, molasses, ½ teaspoon thyme, oregano, and cayenne along with the remaining onion and garlic.
In the bottom of a 5-quart or larger slow cooker (greasing it is optional), pour a thin layer of sauce. Layer half the cabbage, half the meat, and half the sauce, then repeat.
Cook on LOW for 8 hours or HIGH for 4 hours.
Serve hot with optional toppings: sour cream, shredded cheese, sliced green onion.