Wipe meat with a damp towel. Place a 5- to 6-quart pan over high heat and add oil (if desired). Add meat and brown lightly all over, turning as needed, about 5 minutes.
Transfer meat to a bowl. Add onions and 2 tablespoons water to pan. Stir frequently until onions are lightly browned, about 5 minutes.
Add beer, soup mix, and 1 cup water to pan; stir to release browned bits. Add beef and any juice. Bring to a boil, reduce heat, cover, and simmer gently until meat is very tender when pierced, about 3 hours.
Lift meat onto a platter and keep warm. Let stand at least 10 minutes to slice neatly. Meanwhile, if needed, boil sauce, uncovered, over high heat, stirring occasionally, until reduced to 3 cups.