Place the onion, garlic, scallion, ground cumin, beer, water, salt and pepper in the food processor. Place half of the marinade in a large zip lock plastic bag.
Add the pork pieces and let it marinate for 24 hours or for at least overnight in the refrigerator. Store the remaining marinade in a container in the fridge.
Preheat the grill to high. Thread the pork onto skewers and brush with vegetable oil. Brush the pork often with the remaining marinade while it is on the grill.
Grill until browned on all sides, 5 to 7 minutes total. Serve the skewers with a papa salada and small arepa.