1. With a small sharp knife, trim ends of plantains and halve crosswise. Cut a lengthwise slit through skin along inside curve. Beginning in center of slit, pry skin from plantain and slice diagonally into one-inch pieces.
2. Heat 1/4 cup oil in a large skillet over medium-high heat. Carefully place one plantain piece in oil. If it bubbles vigorously, the oil is ready.
3. Working in batches, cook plantains, turning once, until beginning to brown, one to two minutes each side.
4. Reduce heat to low and continue to cook, turning occasionally, until soft and deep golden brown, six to eight minutes.
5. Transfer to a paper towel-lined sheet tray. Season with salt and/or sugar, if desired. Serve warm.