In a 5-qt. slow cooker, combine the first 5 ingredients. Add beef; stir to coat. Cover and cook on low until tender, about 6 hours. , In a small bowl, whisk cornstarch and cold water until smooth; stir into slow cooker. Cover and cook on high until thickened, about 30 minutes. Meanwhile, in a large saucepan, place a steamer basket over 1 in. of water. Place broccoli in basket. Bring water to a boil. Reduce heat to maintain a simmer; steam, covered, until crisp-tender, 3-4 minutes. Stir broccoli into slow cooker. Serve over rice. If desired, garnish with optional ingredients.