Remove the outer leaves of the cabbage, cut the head in half through the core, and set one half aside. Cut out the tough core from the remaining half, then slice into thin shreds until you have about 5 cups. Peel and chop the half onion into small, even pieces. Drain and rinse the kidney beans in a colander, then open the three cans of tomato sauce.
Set a large stockpot over medium-high heat and add the vegetable oil. Once the oil shimmers, add the ground beef and chopped onion. Cook for 5 to 7 minutes, stirring often and breaking the beef into small crumbles, until the beef is well browned with no pink left and the onion is soft and translucent.
Turn off the heat and carefully tilt the pot, or spoon the beef and onion into a colander set over a heatproof bowl, and let the fat drain away completely. Discard the fat in a sealed container or the trash, never down the sink.
Transfer the drained beef and onion mixture into the slow cooker. Add the shredded cabbage, drained kidney beans, and tomato sauce. Pour in the water and drop in the bouillon cubes, then sprinkle in the cumin, salt, and black pepper.
Stir everything well, pressing the bouillon cubes against the side of the slow cooker to help them break apart, until the cubes begin to dissolve and the tomato sauce and water look evenly mixed. Place the lid on the slow cooker and make sure it sits snugly.
Cook on the high setting for 4 hours, or on the low setting for 6 to 8 hours, giving the soup a gentle stir once or twice if you are home. The soup is ready when the cabbage is completely tender and the broth smells rich and savory.
Stir the soup one final time and taste a spoonful of the broth. Add a little more salt or pepper if needed, keeping in mind that bouillon and tomato sauce are already salty. Ladle the hot soup into bowls and serve right away.