Increase heat to medium-high. Add oil; swirl to coat. Sprinkle beef with 1 tablespoon flour, 1/2 teaspoon salt, and pepper. Add beef to pan; cook 10 minutes, turning to brown on all sides. Add beer to pan; bring to a simmer, scraping pan to loosen browned bits. Add onion mixture, stock, remaining 1/2 teaspoon salt, and rosemary to pan; bring to a simmer. Cover, reduce heat, and simmer 2 hours. Turn beef; cover and cook 2 hours or until very tender. Place beef on a plate, and cover with foil.