Step 1

In a large high-sided skillet, heat butter over medium-high heat. Add diced onion, and saute until translucent and beginning to brown, about 6 minutes. Add the prepared mustard greens, and sprinkle with salt and pepper.

Step 2

Toss until greens are just wilted, adding more greens as they will fit into the pan. Toss with lemon juice just before serving.