Rinse the mustard greens thoroughly to remove dirt and grit
Trim off tough stems and any damaged leaves
Chop the leaves and stems into bite-size pieces
Heat oil or butter in a large skillet or pot over medium heat
Add chopped onion or garlic if desired and cook until fragrant
Add the mustard greens to the pan
Stir until the greens begin to wilt
Add a small splash of water, broth, or stock
Cover and cook for 5 to 10 minutes, stirring occasionally
Season with salt, pepper, and optional vinegar, lemon juice, or red pepper flakes
Cook until the greens are tender but still slightly firm
Serve hot as a side dish or mix into other dishes