Heat a large skillet over medium. Add the ground beef, Italian sausage, and chopped onion. Cook, breaking up the meat, until no pink remains and the onion is soft, about 6–8 minutes. Drain off excess fat. Taste and season lightly with salt and pepper.

In the Crock Pot, spread half of the chopped cabbage in an even layer. Spoon half of the meat mixture over the cabbage. Repeat with the remaining cabbage and meat to make two layers.

In a medium bowl, whisk together the tomato sauce, diced tomatoes (with juices), uncooked rice, beef broth, garlic powder, paprika, and a pinch of salt and pepper. Pour this mixture evenly over the layered cabbage and meat — the rice should be submerged so it cooks properly.

Cover and cook on LOW for 6–8 hours. Check near 6 hours: the cabbage should be tender and the rice cooked through. If the rice still feels firm, cook another 30–60 minutes.

When done, uncover and gently stir to combine everything into a cohesive casserole. Serve warm.