Bring a large pot of slightly salted water to a boil. Add greens and cook, uncovered, 2-5 minutes.
Scoop out the greens with a skimmer or slotted spoon into a colander, then rinse under cold water to stop cooking.
Drain well, gently pressing out excess water, and transfer to a bowl.
Meanwhile, heat oil in a medium pot or pan over medium heat.
Add onions and garlic, stirring frequently until tender and fragrant, about 8 minutes.
Add greens and cook covered, stirring occasionally, for 5-10 minutes more. Serve warm!