Warm a large skillet over medium to medium-high heat then cook beef with cumin until browned and crumbly, 5-7 minutes. Drain grease and transfer to a mixing bowl.
Add rice, tomatoes, and Worcestershire sauce to mixing bowl with beef and stir to combine.
Cut the top off each bell pepper and set aside. Deseed the peppers if desired. Fill each pepper with 1/6 meat and rice blend, then transfer to slow cooker. Place the tops back on the peppers.
Pour water around the base of the bell peppers, then cover and cook on low for 5 1/2 - 7 1/2 hours. Top with cheese and cook 30 minutes more.
Notes
If you've got the extra time for chopping, cook 1/4 cup diced onion and 3-4 cloves garlic with the meat for more layers of flavor.