In a large pot of boiling water, cook kale, uncovered, stirring occasionally, until just tender, about 10 minutes. Drain in a colander and set aside.
In a large skillet, heat oil over medium-high heat until hot. Add onion and sauté, stirring occasionally, until softened, 6 to 8 minutes. Add garlic and red-pepper flakes and sauté, stirring, for 1 minute. Reduce heat to medium and add kale. Cook, stirring occasionally, until heated through. Remove from heat and stir in vinegar and salt.
In a large pot of boiling water, cook kale, uncovered, stirring occasionally, until just tender, about 10 minutes. Drain in a colander and set aside.
In a large skillet, heat oil over medium-high heat until hot. Add onion and sauté, stirring occasionally, until softened, 6 to 8 minutes. Add garlic and red-pepper flakes and sauté, stirring, for 1 minute. Reduce heat to medium and add kale. Cook, stirring occasionally, until heated through. Remove from heat and stir in vinegar and salt.
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