Place the scallions, achiote, cumin, onions, garlic and 2 cups of beer and blend until smooth.
Add the marinade evenly over the pork and cover.
Let the pork marinate in the refrigerator for 1 to 2 days.
Pre-heat the oven to 300 degrees F, bake for about 3 hours, turning the pork roast over after 1 hour to brown the other side. Baste with the remaining beer and vegetable oil, about 5 times throughout the cooking time.
Remove from the oven and place on a cutting board. Cover with aluminum foil and allow to rest for 10 minutes. Slice the pork thinly and serve with your favorite side dishes.