Peel green plantains by cutting off the top and bottom. Then slice lengthwise through the skin trying not to cut into the plantain itself. Remove the peel in sections using your fingers or a knife to pry off the first section; work slowly; the peel is quite thick, and you don’t want to break the plantain.
Cut the plantains into approximately ¾ inch thick rounds, straight, or on the diagonal.
Pour the oil into a medium skillet until it is ¼ inch deep. Heat the oil over medium heat until shimmering. You can test to see if the oil is hot enough by dipping a slice of plantain into the oil. Small bubbles should form around the plantain. If big bubbles form, then the oil is too hot and the plantains will brown on the outside before they are cooked in the middle. Meanwhile, cover a plate or work surface with a couple layers of paper towels.
Once the oil is hot enough, place the plantains in the pan with one of the cut sides down, making sure there is a bit of room between each plantain (you will likely have to cook the plantains in batches). Fry the plantains for 3 to 4 minutes until the underside is golden brown, then flip the plantains and continue to fry on the other side for an additional 3-4 minutes until that side is golden brown. Adjust the heat as necessary so the oil is at a gentle simmer. When they are done, they should be light golden brown and you should be able to pierce a plantain with a fork with minimal resistance.
Once the plantains have been fried on both sides, remove them to the paper towel lined surface. While they are hot, use a flat-bottomed bowl, pot, or cup to press down and flatten the plantains until they are about ¼ inch thick. If the plantains break apart while you are doing this, they are not cooked enough and need to be fried for an additional 1 to 2 minutes.
Turn the heat up to medium high. Return the flattened plantains to the pan and fry for 1 to 2 minutes per side until they are golden brown. The oil will be bubbling. Remove the plantains from the oil onto a paper towel lined surface to absorb the excess oil and salt generously on both sides immediately. Serve hot.
Notes
Your plaintains don’t have to be completely green, but if your plantains are completely yellow they are ripe, and will be sweet – great for other dishes, but tostones are traditionally savory – kind of starchy and benefitting from a serious sprinkle of salt while they are still hot.