Partially fill a 4-qt. saucepan with water and bring to a boil. Add greens and salt, stirring until wilted, about 1-2 minutes. Drain and rinse with cold water to stop cooking. When cooled, squeeze greens to remove excess water.
In a large heavy skillet, heat olive oil over medium heat and sauté onion until soft and golden brown, about 5 minutes. Add garlic and stir for 1 minute, not allowing it to brown. Add greens and broth and continue to sauté, sitting occasionally, until broth evaporates and greens are tender about 5 minutes. Add vinegar and pepper to taste just prior to serving.
