Add the greens. Cover the pan and cook until tender. Chard and beet greens will take 8 to 10 minutes; kale, mustard, turnip, and collard greens will take 20 to 25 minutes. Drain the greens well in a colander, pressing to remove excess liquid. Toss with 1 tablespoon butter or olive oil, if desired, and season to taste with salt and pepper.