1Heat oven to 300°F. In small bowl, mix brown sugar, garlic, wine, chili sauce, ketchup and vinegar with whisk until well blended. Set aside.
2Heat 12-inch skillet over medium-high heat 2 minutes. Pat beef brisket dry with paper towel; season with salt and pepper on each side. Sear beef 2 minutes on one side (beef will develop a dark crust). Turn beef; sear on other side.
3Place beef in large roasting pan. Cover with sauce, sliced onions and thyme sprigs. Cover with foil; bake 3 hours.
4Let beef stand 10 minutes. Cut across grain into slices. Serve with cooked onions and pan drippings.