Wash spinach well. Gather spinach, root ends together, into 4 equal bunches and tie each bunch tightly around middle of stems with string. Blanch spinach, 2 bunches at a time, in a 6- to 8-quart pot of boiling salted water until just wilted, 15 to 30 seconds, then transfer with tongs to a large bowl of cold water to cool. Drain spinach and squeeze excess water from each bunch, squeezing well several times. Cut off string and cut spinach crosswise into 1-inch strips, including first inch of stems. Discard tough stem ends.
Stir together sesame paste, miso, water, and soy sauce to form a smooth, thick sauce.
Toss spinach with sauce and sprinkle with sesame seeds.
