For the onions: Preheat the oven to 400 degrees F.
Bring the vinegar, sugar and salt to a simmer in a small saucepan over medium-high heat to dissolve the sugar. Place the onions in a small bowl and pour the hot mixture over them. Toss well to coat. Allow to cool to room temperature for 1 hour before using.
For the chickpeas: Meanwhile, toss the chickpeas with the olive oil, lemon zest, zaatar and salt. Spread on a rimmed baking sheet and roast until golden and crisp, about 30 minutes. Drain well on paper towels.
For the salad: Place the Tuscan kale in a large bowl. Add 1/4 teaspoon salt and massage the salt into the kale. Add the olive oil, vinegar and remaining 1/4 teaspoon salt. Add 1/2 cup of the onions without their liquid, all of the chickpeas, feta and zaatar. Toss to combine and serve.
The remaining onions can be kept in the refrigerator in an airtight container for up to 2 weeks.