Put oven rack in middle position. Preheat oven to 325F. Pat beef dry and rub with 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Heat oil in oven proof, 5-quart, wide heavy pot over medium high heat until hot - not smoking - and brown beef on all sides, about 15 minutes total. Transfer beef to a plate.
Add onions to pot and saute, stirring, until pale golden, about 10 minutes. Add garlic, thyme, rosemary, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring, for two minutes. Add stout and water, bring to a boil, return beef to pot and cover. Braise in oven, turning after one hour until beef is tender - three to three and a half hours total. Let stand uncovered in onion sauce about 30 minutes before serving.