Combine cucumber and salt, tossing well. Let stand at room temperature 20 minutes. Drain and squeeze dry.
Combine cucumber, onions, rice vinegar, sugar, crushed red pepper, and sesame oil. Serve chilled or at room temperature.
Use a mandoline or food processor to slice the cucumber uniformly thin. Salting the cucumber causes it to wilt and draws out excess moisture. It also gives the dish a pickled quality similar to kimchi, Korea's spicy and pungent fermented cabbage condiment.