Kenji's genius method here is to massage the hearty kale with olive oil and a little salt and then let it stand for up to an hour to soften and make it tender yet crisp.
Massage kale with olive oil and 1/2 teaspoon salt in a large bowl, coating all surfaces, about 2 minutes. Let stand at room temperature at least 15 minutes and up to 1 hour.
Rinse onion in a sieve under warm water for 2 minutes. Drain well. Combine onion, sumac (if desired), and sesame seeds in a small bowl. Combine juice, mustard, and garlic in a small bowl.
Add juice mixture and chickpeas to softened kale; toss well. Stir in remaining 1/4 teaspoon salt and pepper. Top with onion mixture.
Adapted from The Food Lab: Better Home Cooking Through Science by J. Kenji López-Alt. Copyright 2015 by J. Kenji López-Alt. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.