What's the technique for frying plantain?

In the Caribbean green plantains are treated much like potatoes. One technique that is common is twice-frying, usually with a soak in water either before the first fry, in between, or both. Soaking plantains allows them to absorb a little water to help steam the inside as it fries, much like one would do for American-style french fries (UK: chips). One example of this technique is Tostones. The Puerto Rican classic cookbook "Cocina Criolla" by Carmen Valldejuli calls for soaking the plantain pieces in room temperature water with salt and crushed garlic for 15 minutes as well as dipping them in water immediately before the second fry. Like many potatoes, green plantains will often turn brown if left out in the air for too long after being peeled, soaking in water also helps prevent this from occurring as well as reducing the amount of free starch on the outside which can cause over-browning.

Elsewhere, both in the Americas and Africa, green plantains are cut very thinly and fried into plantain chips (UK: crisps). These are usually 1-2mm thick and also benefit from presoaking.

If you decide not to fry them you could also boil them in salted water like a potato or boil them in vinegar with onions, garlic, and spices to make an escabeche. You could also partially cook them in salted water or stock then bake them in the oven.

It sounds like you just undercooked them. They're fairly similar to potatoes; if you just chop them up and fry with reasonably high heat on the stove, you're going to brown the outside before the inside is cooked. If you want to cook them purely by frying, you want to use lower heat, and possibly more oil, so that you're efficiently transferring heat to the whole thing, not just the bottom.

I might suggest, though, partially cooking them first, whether it be by baking or steaming or microwaving. That way, when you fry them you're mostly browning and finishing them, not relying on it to cook them the whole way through. If you do bake them, be sure to leave the skin on - they'll sort of steam internally, and it'll come off easily when you're done, at which point you can quickly slice and fry. I know the traditional technique is double-frying, but baking and microwaving require a lot less active work, and scale better if your pans aren't too big.

Another method is to make them into "maduros", a sweet and very tasty treat.

Let the plantains over-ripen; the peels should have lots of black over top of deep yellow. Then slice them roughly 2cm thick, on an angle. Cook in a pan of oil (enough to submerge half a plantain at a time) on medium heat until they turn golden brown. Flip and repeat, then remove from the oil & dab away the excess.

You need to soak them in salted water first to flavor them as well as extract some of the "free starch" as another another person mentioned.

You also cut the plantains too thick. When frying like this, they will come out crispy if thinly sliced but will be hard if the plantain is too thick.

If you want softer plantains, you need to cook them slowly and for a longer period of time. Just like a potato.