When I was growing up in Puerto Rico, meat was always marinated. I don't remember in my house ever cooking a piece of meat that hadn’t been soaked in a acidic sauce of some kind. My dad would marinate a lechon (roast pork) for five days for all of us to enjoy. In all my restaurants and anytime I cook, all of my meats are marinated for at least a day. Most of them, two days. The flavor that the marinade adds to the dish is just unbelievable and it also imparts great tenderness. What you're doing is creating an atmosphere of flavors for the food and if you leave it for at least six hours—or even if you do it for two hours—it just enhances anything. The use of sweet plantains is a very Latin thing: I put them to good use by pureeing them with good quality butter and a lot of cream. The creaminess is really wonderful with the pork.
Make the adobo in a mortar
Slowly stir in the olive oil.
Place the pork tenderloins in a shallow dish or on a baking sheet and brush with the adobo.
Cover tightly and marinate in the refrigerate for at least 30 minutes.
Preheat the oven to 350°F. Season the pork loins with black pepper. Pour about 1/8 inch of olive oil into a large ovenproof skillet to coat the bottom; heat over high heat. Add the loins and cook, turning often with tongs, until browned on all sides, about 5 minutes. Add the remaining smashed garlic clove and the thyme sprigs and spoon the oil over the loins. Transfer the skillet to the oven to cook the pork until an instant-read thermometer inserted in the center of a loin registers 143°F, another 4 minutes. Let stand in the skillet for 3 to 4 minutes.
While the pork is marinating, bring 3 cups water and the salt to a boil in a medium saucepan. Peel each plantain: Cut off the ends, make 1 or 2 lengthwise cuts just through the skin, and remove the peel with your fingers.
(Unlike the green plantains used to make the arañitas, ripe plantains are much easier to peel.) Cut each plantain lengthwise into 4 pieces and cut away the vein from the center (the vein is stringy and unappetizing). Cut into 4-inch pieces.
Add the plantains to the boiling water and cook until soft, about 12 minutes. Drain. Pass the plantains through a ricer into a medium bowl to puree.
In a blender, combine all the ingredients and blend until smooth. You can cover and refrigerate the salsita for up to 2 hours—any longer and it will turn brown.
To serve, slice the loins into 3/4-inch-thick slices. For each serving, place a dollop of the plantain puree in the center of a plate, fan out one-fourth of the sliced pork in front, and drizzle a tablespoon of the salsita on the edge of the meat.