Slice the beef sirloin into thin strips against the grain for maximum tenderness.
Place the beef strips, sliced white onion, and minced garlic into the base of the slow cooker.
In a small mixing bowl, whisk together the beef broth, soy sauce, brown sugar, and grated ginger. Pour this sauce evenly over the beef and onions.
Cover the slow cooker and cook on LOW heat for 4 to 5 hours, or HIGH heat for 2 to 3 hours, until the meat is completely tender.
Approximately 30 minutes before the cooking time is complete, add the sliced green, red, and yellow bell peppers. This keeps them crisp-tender.
In a small bowl, whisk the cornstarch and cold water to create a smooth slurry. Stir the slurry into the slow cooker, cover, and let cook on HIGH for an additional 20 minutes until the sauce has thickened.
Serve warm over prepared white rice or noodles.