Line a large dinner plate with a couple of paper towels and set aside.
Make a long lengthwise cut through the thick skin of each plantain (avoid cutting the flesh) and peel off the skin. Place peeled plantains on a cutting board.
Cut the plantains into 1-inch-thick slices.
Pour 1/2 cup avocado oil into a large skillet and heat over medium-high heat until the oil is hot and shimmering. You can test readiness by carefully touching a plantain slice to the oil—if it immediately sizzles, the oil is ready.
Working in a single layer and without overcrowding, carefully place plantain slices into the hot oil. Fry in batches as needed.
Fry the slices until the side in the oil is golden brown, then flip. Continue cooking, flipping several times, until the other side and the edges are deeply golden brown and the slices are cooked through.
Transfer cooked plantains to the paper towel–lined plate to drain and immediately sprinkle with sea salt.
Serve the fried plantains warm.