There are two kinds of ohitashi. One way is to eat simply boiled greens with drops of soy sauce and katsuobushi. Another way is to soak boiled greens in dashi mixture for hours before serving. This ohitashi is the latter one, but both of them are popular cooking method in Japan.
Most typical greens for ohitashi are broccoli rabe and spinach. But I made using broccoli, the vegetable you can find easily in the United States. The point is to remove excess water well with paper towels after boiling and cooling down the broccoli.
Broccoli is so flavorful when soak up dashi mixture.