Adjust rack to top position and heat broiler to high or oven to 500 degrees. Wash and dry all produce. Halve zucchini lengthwise; cut into long, thin wedges. Halve, peel, and cut onion into 1-inch-thick wedges; mince 1 wedge. Zest and quarter lemon. Halve tomatoes. Roughly chop cilantro leaves and stems.

Heat a drizzle of olive oil in a small pot over medium-high heat. Add minced onion and couscous. Cook, stirring, until onion is softened and couscous is lightly toasted, 2-3 minutes. Add ¾ cup water (1½ cups for 4 servings) and stock concentrate. Bring to a boil, then cover and reduce to a low simmer. Cook until couscous is tender, 6-8 minutes. Keep covered off heat.

Meanwhile, toss zucchini and onion wedges on a baking sheet with a large drizzle of olive oil, sumac, salt, and pepper. Broil or bake on top rack, tossing halfway through, until lightly charred, 10-15 minutes.

While veggies roast, slice grilling cheese into four large pieces (for 4 servings, cut into eight large pieces). Place Za’atar Spice on a plate. Coat cheese in za’atar, pressing to adhere. Heat a drizzle of olive oil in a large, preferably nonstick, pan over mediumhigh heat. Add cheese and cook until browned, 1-2 minutes per side.

Transfer cooked couscous to a large bowl. Stir in tomatoes, lemon zest, half the roasted veggies, half the cilantro, and a squeeze of lemon juice. Season with salt and pepper.

Divide couscous mixture between plates. Top with grilling cheese and remaining roasted veggies. Garnish with remaining cilantro. Serve with remaining lemon wedges on the side.