Trim the ends off 4 green plantains (about 800 g). Make a shallow lengthwise slit through the skin and peel off the skin.
Cut the peeled plantains into 1-inch (≈2.5 cm) thick slices.
Pour all 470 ml cooking oil into a large frying pan or skillet and heat over medium-high until the oil is hot and shimmering.
Working in a single layer (do not overcrowd the pan), carefully add the plantain slices to the hot oil. Fry about 2–3 minutes per side, until they are lightly golden. Use tongs or a slotted spoon to turn them.
Transfer the fried slices to a paper towel–lined plate to drain and cool briefly.
Flatten each fried slice to about half its original thickness using a tostonera or the flat bottom of a glass/jar. (Place the slice between two sheets of paper towel or a clean surface to prevent sticking.)
In a mortar and pestle, crush the 8 garlic cloves with a small pinch of the salt (from the "salt to taste") until you have a garlic paste.
Spread a small amount of the garlic paste onto one side of each flattened plantain.
Reheat the oil if it has cooled. Return the flattened, garlic-topped plantains to the hot oil and fry about 1–2 minutes per side, until crispy and golden brown.
Remove the tostones to a paper towel–lined plate to drain. Sprinkle additional salt to taste and serve hot.