Coat a large saute pan with olive oil and put on medium-high heat. Toss in the garlic cloves and a pinch of crushed red pepper. Cook the garlic until it becomes golden brown and is very aromatic. Remove the garlic and discard.
Add the mustard greens and season with salt, to taste. Cover and cook the greens until they are soft and wilted. Stir in the vinegar. Taste for seasoning and add salt, if needed. Transfer the greens to a serving bowl and serve.