Add the shrimp and tomato sauce and stir well. Cover and simmer, stirring from time to time, until the shrimp is warm and the sauce is well blended, about 5 minutes. Remove from the heat and set aside.
Prepare the plantains: Soak them in warm water in a large bowl (or in the sink) for about 20 minutes; this will soften the skin and make peeling easier. Remove the plantains from the water, dry them, cut the ends off, and cut a slit lengthwise down the middle. Open and remove the peel, using a knife or your hands, as if you were taking a jacket off somebody. Cut them on the diagonal into 1/2-inch chunks.
Add about 2 inches of vegetable oil to another skillet—it should be enough to submerge the plantain pieces—and heat over high heat. Fry the plantain pieces without crowding them in the pan, until golden, 3 to 5 minutes on each side. Remove the plantains to drain on paper towels, but reserve the oil. Repeat with the remaining plantain pieces, if necessary.
Drop the garlic into a pilón or a wooden bowl 2 to 3 inches wide, and mash it with a pestle. (If you don’t have a pestle, use the back of a wooden spoon.) Add a dash of salt. Remove the mashed garlic and set aside.
Take a tablespoon of the hot oil from the skillet and coat the inside of the pilón with it (this will keep the plantain from sticking).
Add 3 or 4 fried plantain slices, along with 1/2 teaspoon of crushed garlic, and mash together until you’ve formed a lining—like a cup—inside the pilón. (If it’s not mashing together well, add more oil.) Once you have created the cup, put 1 or 2 tablespoons of the shrimp mixture in the center (you want to fill it with the shrimp).
Here comes the tricky part: Cover the top of the pilón with a small, flat serving plate, and flip it so that the open side is upside down. Carefully lift off the pilón. It should look like a small mound. To serve, ladle a bit of the shrimp mixture on top.
NOTE: If your plantain cup doesn’t come out easily the first time, try oiling a butter knife and gently scraping it around the outer edges—as you would with a cake mold—so that the plantain doesn’t stick to the pilón.
