Peel the plantains and cut into chunks. (Tip: Microwaving the plantains for 30 seconds makes peeling easier)
Heat the vegetable oil in a deep pan or fryer to 325°F.
Fry the plantain chunks in the hot oil until completely soft inside, about 8 minutes.
Remove the fried plantains and drain on paper towels.
In a large mortar and pestle (pilon), mash the fried plantains with the butter, chopped garlic, and salt until well combined.
Cut the pork butt or shoulder into chunks.
In a bowl, combine salt, pepper, chopped garlic, oregano, rosemary, lime juice, and Sazón. Mix well.
Rub the pork chunks with the seasoning mixture and let it marinate for at least 2 hours.
Preheat the oven to 350°F.
Place the marinated pork chunks in a deep roasting pan and cover with foil.
Bake in the preheated oven for 45 minutes to 1 hour, until the pork is tender. The longer it bakes, the more tender it will be.
After baking, heat a bit of oil in a pan and fry the pork chunks for a couple of minutes to crisp them up.
In a pan, melt some butter over medium heat. Add the sliced red onions, chopped garlic, salt, pepper, and red wine vinegar. Cook until the onions are fully cooked but not caramelized, stirring occasionally.
Serve the mofongo alongside the fried pork Garnish with fresh cilantro.
Top the mofongo with the sautéed onions.