In a large saucepan over medium heat, melt the butter and then whisk in the flour. Cook, while whisking, about a minute. Whisk in the water and continue to cook until mixture begins to bubble. Add the kale and stir until well coated. Add one tablespoon of the za'atar spice, and continue cooking, stirring occasionally, until the kale has wilted and becomes tender, about two minutes. Turn off the heat and fold in the yogurt. Set aside.
Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms and sauté until soft and beginning to brown. Add the onions and cook until soft and translucent, then add the garlic and cook, stirring, for a minute more.
Add the ground lamb, remaining tablespoon of za'atar, and cook, stirring occasionally, until the lamb is browned. Season to taste with salt and pepper.
In a medium saucepan, bring 1 1/2 cups water, 1 tablespoon olive oil and 1 teaspoon salt to a boil. Stir in the dried couscous, turn off the heat and cover. Let the couscous steam for about 5 minutes, then fluff with a fork.
Serve the spiced lamb mixture over the couscous and top with yogurt creamed kale. Sprinkle crumbled feta on top and garnish with chopped pistachios, if desired.