Combine kale leaves with olive oil and 1 teaspoon salt in a large bowl. Toss until evenly coated. Allow to rest at room temperature until kale is lightly wilted, about 1 hour.
Meanwhile, place onions in a medium bowl and cover with cold water. Allow to rest for 15 minutes. Rinse in several changes of water then carefully dry with a salad spinner lined with paper towels. Combine onions with sumac and sesame seeds. Season to taste with salt.
Combine lemon juice, garlic, and mustard in a small bowl. Once kale is wilted, toss kale, lemon juice mixture, and chickpeas in a large bowl. Season to taste with salt and pepper. Serve salad topped with sumac onions.