Mix the onion, garlic, scallion, ground cumin, beer, salt and pepper in a large zip lock plastic bag.
Add the beef and let it marinate overnight or for at least 5 hours in the refrigerator.
Preheat the grill to high. Thread the beef onto skewers and brush with olive oil. Grill until browned on all sides, 5 to 7 minutes total. Serve the skewers with a papa salada and small arepa.