Rapini (broccoli rabe) has a nice light bitter flavor and makes a good combination with the hot kick of karashi (Japanese mustard). I also add some sesame paste and it tastes really wonderful. This can be served either cold or cool and makes a nice appetizer or a side dish. The sauce is quite versatile, so you can enjoy it with different kinds of vegetables such as broccoli, cauliflower, green beans, etc.