Set a rack in the middle of the oven and heat the oven to 425°F. Lightly oil a large rimmed sheet pan or line it with parchment paper for easier cleanup.

In a large bowl, stir together the harissa paste, honey, juice from 1 lemon half, 2 tablespoons of the olive oil, 1 teaspoon of the kosher salt, and 1/4 teaspoon of the black pepper.

Put the chicken thighs into the bowl. Turn them in the harissa mixture until they are well coated.

Set the bowl of chicken aside at room temperature while you start the vegetables. This short rest gives the seasoning a little time to cling to the meat.

Put the cauliflower, carrots, and red onion on the sheet pan.

Drizzle the vegetables with the remaining 1 tablespoon olive oil.

Sprinkle the vegetables with the remaining 1/2 teaspoon kosher salt and the remaining 1/4 teaspoon black pepper.

Toss the vegetables right on the sheet pan until they are coated, then spread them into one even layer so they can roast instead of steam.

Put the sheet pan in the oven and roast the vegetables for 10 minutes. They should look a little drier on the surface and just start to soften at the edges.

Take the sheet pan out of the oven and use a spatula to turn the vegetables.

Make 4 open spaces on the sheet pan so the chicken can sit directly on the hot pan instead of on top of the vegetables.

Place the chicken thighs in the open spaces, smooth side up. Scrape any harissa mixture left in the bowl over the chicken.