Heat the oven to 425°F. Set a large rimmed sheet pan on the counter and line it with parchment or foil if you want easier cleanup.

In a large bowl, stir together 2 tablespoons of the olive oil, 2 teaspoons of the shawarma seasoning, 1 teaspoon of the salt, and 1/4 teaspoon of the black pepper.

Add the chicken thighs to the bowl. Turn them well so the seasoning coats both sides, then set the bowl aside while you start the vegetables.

Put the sweet potatoes on the sheet pan. Drizzle them with 1 tablespoon of the olive oil and sprinkle with 1 teaspoon of the shawarma seasoning, 1/4 teaspoon of the salt, and a few grinds of black pepper.

Spread the sweet potatoes into a single layer on the sheet pan. Roast them for 10 minutes so they can get a head start.

While the sweet potatoes roast, put the cabbage and red onion in a second large bowl.

Drizzle the cabbage and onion with the remaining 1 tablespoon olive oil. Sprinkle with the remaining 1/4 teaspoon salt and the remaining 1/4 teaspoon black pepper, then toss until the vegetables look lightly coated.

Take the sheet pan out of the oven. Use a spatula to turn the sweet potatoes and move them toward the outer edges of the pan.

Add the cabbage and red onion to the center of the sheet pan. Spread them out so they are in an even layer and not packed into one thick pile.

Place the chicken thighs on top of and between the vegetables on the sheet pan. Squeeze the juice from one lemon half over the chicken and vegetables.

Return the sheet pan to the oven. Roast for 15 minutes.

Pull the sheet pan out again. Turn the cabbage and onion with a spatula so new edges touch the hot pan, and flip any chicken pieces that still look pale on top.