Heat oil in a large frying pan over medium heat; cook and stir onion in the hot oil until slightly softened, 1 to 2 minutes. Add plantains; cook, stirring occasionally, until plantains are crisp and lightly browned, 5 to 7 minutes. The plantain has a tendency to stick together, so the more you stir the more the individual pieces will separate.
Fold raisins into plantain mixture and cook until raisins are plump and heated, 1 to 2 minutes. Season with salt.