Brown the ground beef in a skillet on the stove top. Drain.
Add the beef, cut up bell pepper tops, rice, 1 cup of the marinara sauce, onion, salt, garlic powder, thyme and oregano to a medium sized bowl. Stir.
Stuff the peppers with the filling you just made. If there is any leftover filling add it to the bottom of the slow cooker.
Add remaining marinara sauce to the slow cooker.
Add the stuffed peppers in the slow cooker. I try not to let them rest on the sides of the slow cooker, but most of the time they end up falling over later.
Cover and cook on LOW for 5 hours without opening the lid during the cooking time.
Video
Notes
Tempted to use raw meat instead of browning? Raw meat in stuffed peppers does work well, but the texture will be more like a meatloaf than a crumbly filling.