Urban Organic

Cook kale in a 6-quart pot of boiling salted water, uncovered, stirring occasionally, until just tender, about 10 minutes, then drain in a colander.
Heat oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté onion, stirring occasionally, until softened, 6 to 8 minutes. Add garlic and red pepper flakes and sauté, stirring, until garlic is fragrant, about 1 minute. Reduce heat to moderate, then add kale and cook, stirring occasionally, until heated through. Remove from heat and stir in vinegar and salt.