This is a typical Japanese dish for spring. The dressing is simply a mixture of mustard and soy sauce, which goes perfectly well with broccoli raab. In Japan it's made with "wagarashi" or Japanese mustard, but I used Dijion mustard (as I live in Italy and wagarashi is not available) and it works as well.
Cut the broccoli raab in bite-sized pieces. Put a cup of water and the broccoli raab in a pot. Cover, turn on the heat to medium, and let it come to boil. Steam-boil it for 1 to 2 minutes. Drain, let it cool down, and squeeze out the water. Set aside.
Right before serving (otherwise it becomes soggy), add the broccoli raab in the bowl and mix well with the sauce.