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 with Currants, Lemon and Olives and Kale with Shiitake Mushrooms. More Recipes for Greens
In a small saucepan, bring the cream to a boil. Simmer over moderately low heat, stirring often, until reduced to 1/3 cup, about 7 minutes.
Meanwhile, in a large pot of boiling water, cook the kale until tender, about 5 minutes. Drain the kale, and when it's cool enough to handle, gently squeeze out the 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 curry powder and cook over moderate heat, stirring, until fragrant, about 3 minutes. Stir in the chicken broth and water chestnuts and bring to a boil. Add the kale and reduced cream and toss. Cover and cook over moderate heat, stirring a few times, until the kale is sizzling, about 3 minutes. Season with salt and pepper and serve.
The kale can be refrigerated overnight; reheat gently, adding a few tablespoons of chicken broth as needed.