Kosher salt and fresh ground black pepper to taste
1-2 teaspoons of red wine vinegar
Process
Clean the greens very well with lots of cold running water - submerge and rinse (repeat 3x). Remove stems and cut into 1-1.5 inch strips.
In a large pot, add a bit of water and add greens (you may need to do this batches). Season with salt and pepper and cook for 3-5 minutes. Remove greens and water to a bowl. In the some pot, add oil and onions (and salt and pepper) and cook until onions are tender and a bit caramelized - about 8-10 minutes.
Next, add garlic and red pepper flakes and cook 3-5 minutes. Bring greens to the pot and heat through and add red wine vinegar. Cook for another 3-5 minutes. Serve hot or cold.
Recipe by Scordo.com - Italian Food Recipes and Lifestyle at http://www.scordo.com/recipe/mustard-greens-recipe.html