I had never cooked (or even heard of!) this vegetable so I searched for a recipe and found this one which tasted as good as it sounded:
Original recipe: Penne with Senposai and Feta
There was no photograph so I followed the recipe as written. I made a triple recipe and used 1 large onion. The next time I make it, I'll double the amount of senposai because I prefer a higher greens-to-pasta ratio.
Pasta with Senposai, Feta, and Olives
1 pound penne or other short chunky pasta
1.5 pounds senposai
1/4 c. EVOO, divided
1 shallot or small onion, finely chopped
2 cloves garlic, minced
pinch red pepper flakes
4 oz feta cheese, crumbled
1/2 c. pitted kalamata olives
1/4 c. chopped fresh parsley
salt and freshly ground black pepper
Bring a large pot of water to boil. Add a generous pinch of salt. Fill a large bowl with cold water.
Wash the senposai leaves, then remove the stem and center ribs (I used my hands to 'strip' them). Chop the leaves into 1/2" x 2" pieces. Blanch in the boiling salted water for 5 minutes, until they're completely wilted but still bright green. Remove with a slotted spoon or tongs to the bowl of cold water to stop the cooking process. When the greens are cool, remove them from the cold water and using your hands squeeze as much liquid from them as possible. (Can be prepared ahead to this point. Refrigerate cooked greens.)
Bring the water used to cook the leaves back to boil (top it off if necessary), add the pasta and cook until done. Drain.
While the pasta is cooking, heat 2 T. EVOO in a large skillet or saute pan. Add onion and pepper flakes and saute on medium heat until onion is translucent, about 5 minutes. Add garlic and senposai leaves, breaking up the squeezed handfuls. Saute stirring often, until heated through, another 5 minutes.
Add drained pasta to skillet and stir to combine and reheat pasta. Add olives, feta, parsley and remaining 2 T. EVOO. Season to taste with black pepper or cayenne (and salt, which really shouldn't be necessary unless you didn't salt the cooking water enough.)
Serve warm or cold.