Start by combining all the marinade ingredients in a large resealable bag. Add the beef slices first, followed by the onion rings, beef stock, olive oil, vinegar, garlic, Sazón seasoning, salt, and oregano. Seal the bag and gently massage it to ensure all the meat is evenly coated in the marinade. Place the bag in the refrigerator and allow the meat to marinate for at least 4 hours. If possible, leave it overnight or up to 2 days for maximum flavor.

Once the beef is fully marinated, heat a large, deep skillet over medium-high heat. There’s no need to add extra oil as the marinade includes plenty of olive oil to prevent sticking.

Carefully pour the entire contents of the marination bag into the skillet, including the beef, onions, and all the liquid. Bring the mixture to a boil, stirring occasionally to distribute the flavors evenly. Once boiling, reduce the heat to low and cover the skillet with a lid. Simmer for about 40 minutes, periodically giving it a stir to ensure nothing sticks to the bottom.

After 40 minutes, check the meat for tenderness with a fork. The beef should be soft and easily break apart without too much resistance. If it’s not fully tender yet, allow it to simmer for an additional 5-10 minutes.

Once the meat is fork-tender, your Bistec Encebollao is ready to be served. Spoon it generously over a bed of rice, letting the succulent juices drizzle over the grains. Top with a few extra onion rings for added flavor and garnish, if desired.