Preheat the oven to 425°F then line a baking sheet with parchment paper. Toss the sweet potatoes with 2 tablespoons of high heat oil and 1 tablespoon za'atar then transfer to the sheet. Roast in the oven for 25 minutes, or until golden on the edges.
Meanwhile, prepare the dressing by combining the yogurt (or tahini), lemon juice, and minced garlic in a large bowl. Whisk until smooth. If using tahini, you may need a splash of water to thin to desired consistency. Add the chopped kale to the bowl then use your hands to massage it with the dressing.
Next toast the walnuts by placing them on a baking sheet and baking in the preheated oven for 5-7 minutes, until light brown and fragrant. Check on them while they are cooking to make sure they don't burn. Remove and set aside.
When ready to eat, distribute kale into four separate bowls then top with roasted sweet potatoes, toasted walnuts, chopped kalamata olives, watercress, pickled shallot and crumbled feta. Serve immediately and enjoy!