Put a large pan of lightly salted water on to boil. Dip the bok-choy in the rapidly boiling water for 2 minutes then drain and refresh in a bowl of cold water. Drain thoroughly as soon as cold and gently squeeze dry. Cut the bok-choy in half longitudinally and arrange on a serving platter. Mix the soy, oil and lemon juice, and use this as a dressing for the greens.