Wash the collards thoroughly and bunch it together. Take each bunch, roll it tightly, and cut in crosswise into thin strips. Wash the strips and drain them thoroughly. Heat the oil in a large, heavy skillet over medium heat, then cook the onion and garlic, stirring them until they are lightly browned. Add the kale strips and cook, stirring for 5 minutes so that the greens are soft, but retain their bright green color. Serve hot.