Mix the beef with the garlic powder, salt and black pepper in a bowl. Mix to combine.
In a large skillet, heat the oil on medium-high heat. Cook the beef on all sides for 5 minutes. Transfer the beef along with any juices to a 6-quart slow cooker.
Add the onions to the skillet that you used for the beef. Turn the heat up to high. Add 1/4 cup of water and stir to remove any browned bits from the pan. After 3 minutes, saute the onions until they are softened and slightly browned. Once the liquid is absorbed, transfer the onions to a slow cooker along with the beef.
Add the green and red bell-peppers (see the recipe note) to the slow cooker. Mix together the honey, Worcestershire sauce and honey in a small bowl. Put the mixture into a slow cooker.
Cook the beef on LOW for 6-7 hours or until tender.
To make a slurry, combine the remaining 2 teaspoons cornstarch and the 2 tablespoons of water in a small bowl. The slurry should be added to the slow cooker. To thicken the sauce, turn the slow cooker on HIGH. Cover it and cook for 10 minutes. Although the pepper beef will be still a little bit saucy, it should become a thick, rich gravy. Stir once more. Add salt, soy sauce, or red pepper flakes to your taste. Serve warm with brown rice, prepared brown rice, and green onions.