Put an inch of water in the pot, lower the steamer basket in it, and add all of the spinach.
Put the pot on the stove, cover it, and turn the heat to high.
When steam starts coming out of the pot, set a timer for 3 minutes. After 3 minutes, turn the heat off, uncover the pot, and leave the spinach to cool completely.
Meanwhile, whisk together the peanut butter, vinegar, soy sauce, honey, and warm water until smooth and creamy, adding more water if you need to.
When the spinach is cool enough to handle (about 30 minutes), lift it out of the steamer basket by the handful, and squeeze it to get rid of the extra moisture. Divide the spinach into 4 bowls, drizzle a little dressing over each, and top with the sesame seeds.