This is the quintessential namul recipe. Most Koreans use spinach, but I like the flavor and slight bite of watercress. You can use whichever greens look fresh at the market.
Ingredients
1pound (455g) watercress or spinach, tough stems removed
Bring a pot of lightly salted water to a boil and prepare a bowl of lightly salted ice water. Blanch the watercress in the boiling water for 45 seconds, then immediately remove it to the ice water. Move the greens around with your clean hands until cooled. Drain them immediately and squeeze all the water out with your hands.
Roughly chop the watercress and transfer it to a mixing bowl. Add the soy sauce, fish sauce, if using, sesame oil, garlic, scallions, sesame seeds, and black pepper to taste. (You can also add more fish sauce, soy sauce, etc., to taste.)
Serve at room temperature or chilled. It will keep in the refrigerator for 2 days, but it is better eaten within a day.