Add the garlic, Ádobo, Sazón, peppercorns, and oregano to a pilón or a small blender. Pound the spices until a thick paste forms.
Pour the white vinegar into the pilón. Use the pestle to stir the marinade together. Pour the marinade into a large mixing bowl.
Add the steaks to the marinade in the bowl, turning them in the marinade a few times to coat them well. Cover the bowl with plastic wrap and marinate the meat, in the refrigerator, for 2-3 hours (or overnight).
Finish the Bistec Encebollao
In a large frying pan, heat the oil over medium heat. Once the oil begins to shimmer in the pan, add the sliced onions. Sauté the onions for 5-6 minutes over medium heat, or until they begin to wilt and take on a little color.
Once the onions are soft, use a set of tongs to move them from the pan to a separate bowl or platter.
Drain all but a tablespoon of the flavored oil from the pan. Return the pan to the stove and increase the temperature to medium-high.Lay the bistecs into the hot oil, then pour the marinade over them in the pan.
Sear the meat for 3 minutes on one side. Flip the steaks over and sear the meat on the second side for 4 minutes.
During the final minute of cooking, return the sautéed onions to the pan. Turn the heat off and- if you have a lid- cover the pan to allow the onions a chance to warm through.