Cleaning mustard stems can be tough. So discard them and keep only the leaves. Fill up a large container with water. Allow the greens to soak for 2-3 minutes in water to remove any impurities. After that, gently remove the greens and dump the water that contains the impurities. Repeat this process at least three to four times more till you have clean water. Now is the time to use the greens. This is something I like to practice a day in advance. When I'm ready to make saag, it saves me a lot of time.