While heating a large skillet over medium-high heat, chop the onion into bite-sized pieces and mince the garlic cloves. Chop the cabbage head into roughly 1-inch squares—this uniform size ensures everything cooks evenly in the slow cooker. Once the skillet is hot, add the ground beef and cook, breaking it apart with a spoon, until browned and cooked through, about 8-10 minutes. Drain any excess fat from the meat using a colander or by tilting the pan and pressing with a spoon to remove the liquid. This keeps the casserole from becoming greasy.

Add the chopped onion and minced garlic to the cooked meat and sauté for about 2 minutes until fragrant. Stir in the paprika, smoked paprika, thyme, and a generous pinch of salt and black pepper, cooking for another 30 seconds to bloom the spices. Pour in the tomato sauce, water, half can of condensed tomato soup, and the diced tomatoes (including their juices). Stir well to combine, then add the rice and stir until it's evenly distributed throughout the sauce. I like to add the rice directly to the meat mixture so it absorbs all those flavors as it cooks.

Add half of the meat-rice mixture from Step 2 to the bottom of a crockpot (approximately 6-quart capacity). Spread half of the chopped cabbage evenly over the meat layer, then repeat with the remaining meat mixture and remaining cabbage. This alternating layer structure helps the cabbage cook evenly and allows flavors to meld throughout the cooking process. In a small bowl, whisk together the remaining condensed tomato soup, remaining tomato sauce, and water, then pour this mixture evenly over the top of the layered casserole. The liquid will seep down through the layers as it cooks.

Cover the crockpot and cook on low for 4 to 6 hours, depending on your slow cooker's intensity and how soft you prefer the vegetables. The cabbage should be fork-tender and the rice fully cooked through. About halfway through cooking, if you'd like, you can give everything a gentle stir, though this isn't strictly necessary. I find that letting it cook undisturbed gives you the best results, with less risk of breaking apart the delicate layers.

Once the casserole is fully cooked, sprinkle the shredded mozzarella evenly over the top, covering as much surface area as possible. Cover the crockpot and let it sit for about 10 minutes to allow the cheese to melt completely. The residual heat will melt the cheese beautifully without overcooking the vegetables beneath. If you shred your cheese from a block rather than using pre-shredded, it will melt more smoothly and create a better texture across the casserole.