Adjust rack to top and middle positions and preheat oven to 450 degrees. Wash and dry all produce. Trim and quarter zucchini lengthwise; cut crosswise into 1-inch pieces. Core, deseed, and dice bell pepper into 1-inch pieces. Halve, peel, and slice shallot into 1/2-inch-thick wedges. Trim and thinly slice scallions, separating whites from greens. Zest and quarter lemon.

Toss zucchini, bell pepper, and shallot on a baking sheet with a drizzle of olive oil, Tunisian Spice, salt, and pepper. Roast on top rack until browned and tender, 18-20 minutes.

Meanwhile, slice each pita into 6 triangles; rub all over with a large drizzle of olive oil until evenly coated. Spread out on a second baking sheet. Sprinkle on both sides with salt and half the garlic powder (all the garlic powder for 4). Bake on middle rack until crisp and browned at edges, 10-12 minutes.

Heat a drizzle of olive oil in a small pot over medium-high heat. Add scallion whites and cook until slightly softened, 30 seconds to 1 minute. Add 3/4 cup water (11/2 cups for 4 servings), stock concentrate, and a large pinch of salt. Bring to a boil, then stir in couscous; cover and remove pot from heat. Keep covered off heat until ready to serve.

In a small bowl, combine yogurt, sour cream, feta, a drizzle of olive oil, and a squeeze of lemon juice to taste. Season with salt and pepper.

Fluff couscous with a fork; stir in lemon zest and season with salt and pepper. Divide between plates; top with roasted veggies and sauce. Sprinkle with scallion greens. Serve with pita chips and any remaining lemon wedges on the side.