Cut the mustard greens crosswise into pieces ½” wide. Slice into 2” long sections. Combine with the scallions in a large bowl.  Mix the salt, vinegar, and 4 c of water in a saucepan.  Bring to a boil. Cool until warm to touch.  Pour over the greens. Place a small plate or other dish on top of the greens to keep them immersed in the salt solution.  Add a weight on top of the plate if necessary. Let stand at room temperature for 2-3 days, until the cabbage is yellowish and sour. Eat, or jar and refrigerate. Refrigerated pickles will keep for 4 weeks.