Cut the head and tail from the mackerels and cut the body of each one into 3 chunks. Cover the fish pieces with the juice of 1/2 lime and leave them in the refrigerator to marinate for around 30 minutes.
Cut the tips off of the bananas, and quarter (lengthwise, then crosswise), or cut through their skins down to the edges, north, south, east, and west, and then steep them in a bowl covered with just-boiled water for 15 to 20 minutes.
Set the mackerel in a medium saucepan, and cover with water. Bring to a boil, then reduce to a simmer, and cook, covered, until completely tender, about 6 to 8 minutes. Lift the fish out from the cooking liquid with a slotted spoon. Remove, and discard the bones and skin, if desired, and break the flesh into good meaty flakes. Sprinkle the chunks with the allspice, the remaining lime juice, and season with salt, and pepper, to taste.
Remove the skin from the bananas. It isn't easy, and you'll need a small, sharp knife to help you at times, but try to pry it off with your hands. Now put the bananas into a pan with enough water to cover. Add 1 tablespoon oil to the water, and bring to a boil. Cook the bananas, covered, on a rolling boil until tender, around 15 minutes.
Pour the remaining 2 tablespoons oil into a large skillet over medium heat. Gently fry the onions until softened, then add the tomatoes, garlic, chile, thyme, and season with salt, and pepper, and cook until everything has softened. Add the mackerel, gently stir, and heat through.
Lift the bananas out of their water with a slotted spoon. You'll notice brown pieces on them. Though most will be in the water, which will also be brown.
Brush off any brown bits and cut the banana into diagonal slices. Stir the banana into the mackerel and serve sprinkled with the cilantro and accompanied by lime wedges.