Coarsely chop the kale leaves. Rinse them, but do not dry.
Heat the oil in a large sauté pan over medium heat until shimmering. Add the garlic and red pepper flakes, if using, and cook, stirring, until fragrant, about 1 minute (do not let the garlic brown).
Add the kale a handful at a time, stirring after each addition so that it starts to wilt, until all of the kale is added. Stir in the salt and pepper. Cover and cook, stirring occasionally, until the kale is just tender, about 5 minutes. Remove from the heat, stir in the lemon juice, and serve.