How to Make It
Bring a large stockpot filled with water to a boil over high. Working in 2 batches, add greens to boiling water; cook, stirring constantly, until just tender, about 30 seconds. Remove greens from boiling water using a fine wire-mesh strainer, and plunge into a bowl filled with ice water to stop the cooking process.
Let greens chill in ice water 5 minutes. Remove greens from water, and transfer to a bowl.
Working in 2 batches, place greens in a clean kitchen towel, and squeeze firmly over sink to remove as much liquid as possible.
Transfer greens to an airtight container, and cover with a damp paper towel until ready to use. (You may store greens in airtight container, covered with paper towel, in refrigerator up to 3 days.)
Ten minutes before serving, stir together master dashi, soy sauce, and salt in a medium bowl. Gently pull apart greens, and add to dashi-soy mixture, pressing to submerge. Let soak 10 minutes. Remove greens from dashi-soy mixture, letting excess drip off (greens should still be juicy). Discard remaining dashi-soy mixture. Sprinkle lemon zest evenly over greens.