In a skillet over medium heat, brown the ground beef until fully cooked. Drain any excess fat.
In a bowl, combine the cooked beef, diced bell pepper tops, cooked rice, 1 cup of marinara sauce, diced onion, salt, garlic powder, thyme, and oregano. Mix well.
Fill each bell pepper with the beef and rice mixture. Place any extra filling on the bottom of the slow cooker.
Pour the remaining marinara sauce over the stuffed peppers in the slow cooker.
Carefully position the stuffed peppers in the slow cooker, trying to keep them upright.
Cover the slow cooker and set to cook on LOW for 5 hours. Resist the urge to lift the lid during cooking to maintain heat and moisture.