Start the barley: Heat the oven to 425°F with a rack in the middle. Set a medium saucepan on the stove.

Rinse the pearl barley under cold water in a fine-mesh sieve until the water runs mostly clear.

Put the barley into the saucepan with 3 cups water and 1/2 teaspoon kosher salt. Bring it to a boil over high heat.

When the water boils, lower the heat to medium-low and cover the saucepan partway. Simmer the barley for 28 to 32 minutes, until the grains are tender but still a little chewy.

Roast the vegetables: Put the cauliflower and red onion on a large sheet pan. Add 2 tablespoons olive oil, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper, then toss well and spread everything into one layer.

Slide the sheet pan into the oven and roast for 15 minutes.

Put the drained chickpeas in a medium bowl. Add the harissa paste and 1 tablespoon olive oil, then stir until the chickpeas are evenly coated.

Take the sheet pan out of the oven. Add the chickpeas to the pan, stir them in with the cauliflower and onion, and spread everything back into one layer.

Return the sheet pan to the oven and roast for 10 to 12 minutes more, until the cauliflower is browned at the edges and the chickpeas are hot and slightly crisp on the outside.

Make the yogurt sauce: Put the Greek yogurt, half of the crumbled feta, the garlic, the lemon zest, and 1 tablespoon lemon juice in a small bowl. Stir until the sauce looks creamy and well mixed.

When the barley is tender, drain off any extra water in a colander if needed, then return the barley to the saucepan.

Add the remaining 1 tablespoon olive oil and 1 tablespoon lemon juice to the barley. Stir well, then taste and add a little more salt and black pepper if it needs it.