Bring water to a boil, add mustard greens, cover the pan and boil gently until tender, about 5 minutes. Drain the greens and discard the water.
Melt butter in a large skillet, add onions, garlic and red pepper flakes and cook 3-5 minutes, until onion is tender. Add cooked greens, lemon juice, pepper and salt. Stir to blend all ingredients and continue cooking until heated through. Serve at once as a vegetable side dish.
Photo by Maryam Nikkhah, vcfoto.com