To make the feta sauce, stir together feta, yogurt, lemon juice, and zest in a small bowl. Add enough water to thin to desired consistency.
To prepare the couscous, bring broth to a boil in a medium saucepan. Add couscous. Reduce heat and simmer, covered, until couscous is tender and broth is absorbed, 8 to 10 minutes. Fluff with a fork. Add pomegranate seeds, pistachios, mint, and baharat. Toss to combine.
Preheat an outdoor grill to medium heat (325 to 375 degrees F (160 to 190 degrees C)). Sprinkle steak cubes with baharat; toss to coat. Thread steak, bell pepper, tomatoes, onion, and mushrooms onto eight 10-inch metal skewers (or bamboo skewers that have been soaked in water at least 30 minutes).
Grill kebabs, turning once or twice, until vegetables are tender and meat is just pink in center, or until desired doneness, 8 to 12 minutes. Serve kebabs with couscous and feta sauce on the side.
