Slice beef into small strips. If you're having any problems making the strips thin, you can place it in the freezer for about 15 minutes. This helps the meat firm up a little and makes it easier to get thin slices. Place sliced beef in bowl. Sprinkle beef with garlic powder, salt and black pepper. Toss to coat. Heat oil in a large skillet over medium high. Add the beef and cook on all sides until the beef is lightly browned, about 3-5 minutes. Transfer the beef and any cooking juices to slow cooker. (Note: This step is for flavor, and can be skipped if you're in a hurry. However, we don't recommend skipping it since it will add more flavor to the dish if you brown the beef first) Add tomatoes, onions, and bell peppers to slow cooker. *(See note on bell peppers below)
In a small bowl, mix water, cornstarch, soy sauce, ground ginger, Worcestershire sauce, brown sugar and red pepper flakes (optional). Pour over beef and pepper mixture in crockpot.
Stir to combine.
Cover and cook on LOW for 5 to 6 hours, until the beef is tender, or on high for 2.5-3 hours. Prior to serving, determine if you want a thicker sauce. If so, see note*
Serve hot over white rice, brown rice, cauliflower rice or egg noodles