In a large skillet over medium-high heat, cook the ground beef and chopped onion until the beef is browned and onions are translucent. Drain excess grease.
Transfer the beef mixture to a 6-quart slow cooker.
Add the chopped red, orange, and green bell peppers, sliced carrots, and cubed potatoes to the slow cooker.
In a separate bowl, whisk together the beef broth, tomato sauce, Worcestershire sauce, soy sauce, minced garlic, Italian seasoning, salt, and pepper.
Pour the liquid mixture over the ingredients in the slow cooker and stir gently to combine.
Cover and cook on Low for 6-8 hours or on High for 3-4 hours, until vegetables are tender.
To thicken the stew (optional): In the final 30 minutes of cooking, whisk the cornstarch and cold water in a small bowl to create a slurry. Stir into the stew and cook on High until thickened.
Taste and adjust seasoning with salt and pepper before serving.