Boiling kale before sautéing it is key to making it tender. If the stems and center veins are very tough, Marcia Kiesel advises removing them: Fold the kale leaves in half with the vein side out, then pull up on the stems. Other creative ideas for braised greens are Kale and Water Chestnuts in Curried Cream and Kale with Shiitake Mushrooms. More Recipes for Greens
In a small bowl, cover the dried currants with hot water. Let stand until softened, about 5 minutes; drain.
Meanwhile, in a large pot of boiling water, cook the kale until tender, about 5 minutes, then drain. When the kale is cool enough to handle, gently squeeze out any excess water. Coarsely chop the kale.
In a large, deep skillet, melt the butter in the olive oil. Add the onion and cook over moderately low heat, stirring occasionally, until softened, about 7 minutes. Add the chicken broth and blanched kale and toss thoroughly. Cover and cook over moderate heat, stirring a few times, until the kale is hot and sizzling, about 3 minutes. Stir in the currants, olives, lemon zest and lemon juice and cook, stirring constantly, until heated through. Season with salt and pepper and serve.
The kale can be covered and refrigerated overnight; reheat gently over moderate heat, adding a few tablespoons of chicken broth as necessary.