Combine garlic, 2 tablespoons oil, pepper, rosemary and thyme in small bowl. Rub over beef tenderloin.
Heat remaining oil in flameproof roasting pan on stove top over high heat; swirl to coat pan. Sear tenderloin in pan 1 minute per side.
Roast in heated 375°F oven for 35 minutes for medium-rare. Remove to platter; keep warm.
Add butter and onion to roasting pan; cook over medium heat, scraping up browned bits, 7 minutes, until onion is softened. Add port, broth and meat juices from platter; cook 8 minutes.
Shake flour and water in small jar to blend. Stir into pan; cook, stirring, 2 minutes or until thickened slightly. Slice tenderloin. Serve with sauce.