Blend the onions, garlic, scallions, beer, vinegar and orange juice to obtain a puree.
Mix the puree with the cumin, dried basil, cilantro, oregano, thyme, salt and pepper.
Make incisions in the pork loin and add the marinade, rub the pork loin with the marinade trying to get it in to the incisions.
Let the pork marinate for at least 24 hours, turning and rubbing it with the marinade about every 6 hours.
Preheat the oven to 300 F °. Bake for about 3 hours, turning the roast every hour.