In a large skillet over medium-high heat, cook the ground beef, breaking it apart with a spoon until no pink remains. Drain off most of the grease, leaving just a small amount for flavor.

Add the diced onion to the same skillet with the cooked beef. Stir and cook for 3 to 5 minutes until the onion softens and turns translucent. Remove from heat.

Lightly grease your 4 to 6-quart slow cooker. Add the rinsed uncooked rice to the bottom and spread into an even layer.

In a bowl, whisk together the condensed tomato soup, water, salt, and black pepper until smooth. Pour this mixture evenly over the rice, gently nudging the rice so it is fully submerged.

Spoon the beef and onion mixture over the top. Give everything a gentle stir to distribute without packing the rice down.

Cover and cook on LOW for 3½ to 4½ hours. Resist the urge to lift the lid during the first 3 hours.

Once the rice is tender and the liquid is absorbed, stir gently from bottom to top. Taste and adjust seasoning. Serve hot straight from the slow cooker.