Trim fat from meat. Season all over with salt and freshly ground pepper. Heat 2 tbsp oil over medium-high heat in Dutch oven or large heavy pot; add beef and brown all over.
Combine garlic, soy sauce, brown sugar, gingerroot, onion, vinegar and water in 4 cup bowl. Pour over beef; bring to a boil. Cover and transfer to 325°F (160°C) oven; cook for 1-1/2 to 2 hours, until meat is fork-tender.
Bok Choy: Heat remaining 1 tbsp sesame oil in large skillet over medium-high heat; add bok choy and sauté for 2 to 3 minutes, flipping occasionally. Add HALF of the cooking sauce and stir in HALF of the cornstarch mixture; cover and cook until simmering and sauce thickens slightly, about 3 minutes, stirring occasionally. Garnish with sesame seeds or cashews.
To serve, remove beef to platter; keep warm. Stir in remaining cornstarch mixture into remaining cooking sauce; heat over high heat for 2 to 3 minutes, until bubbling and thickened slightly; spoon over beef. Portion beef and bok choy onto plates, along with steamed rice or noodles if desired.