Cut off the tops of the peppers and remove the ribs and seeds from inside and discard. Set the cleaned peppers aside.
In a medium microwave-safe bowl mix the onions, garlic, and olive oil together. Place into the microwave and heat for 5 minutes on high. Be careful when you take this out of the microwave because the bowl will be hot. Set aside to cool for 3-5 minutes.
In a large bowl mash together the bread and milk until a paste forms. Then add in the ground beef, parmesan cheese, brown rice, ¾ cup shredded cheese, ¼ cup spaghetti sauce, and the onion and garlic mixture, salt, and pepper. Mix together until all of the ingredients are evenly distributed throughout the ground beef.
Spread the remaining sauce onto the bottom of the slow cooker insert.
Stuff each pepper with the ground beef mixture and then place it into the slow cooker insert standing up on top of the sauce.
Put the lid on and set the slow cooker to cook on low 4-6 hours. The ground beef should be cooked thoroughly. My peppers cook fully at 6 hours.
With about 5 minutes left add the remaining shredded cheese to the peppers and place the lid back on. Serve after the cheese has melted.