Heat a large iron or stainless steel skillet over medium heat. Add enough olive oil to coat the bottom of the pan. Add onions and sauté until soft, but not browned.
Add the chopped kale to the pan and cook, stirring frequently, until the kale has turned bright green. Pour the lemon juice over the kale and cover the skillet tightly. Cook 15 to 20 minutes, stirring occasionally, until the kale is dark green and tender. 4 servings.