Heat the oven: Heat the oven to 425°F with a rack in the middle. Set out a large rimmed sheet pan.

Mix the mustard glaze: In a large bowl, whisk together the Dijon mustard, honey, 2 tablespoons of the olive oil, the juice from half of the lemon, the garlic powder, 3/4 teaspoon of the salt, and 1/4 teaspoon of the black pepper until smooth.

Hold back some glaze: Spoon 2 tablespoons of the mustard glaze into a small bowl and set it aside for finishing later. Leave the rest of the glaze in the large bowl for the chicken.

Arrange the vegetables: Put the cabbage wedges, carrots, and red onion wedges on the sheet pan. Spread them into one layer so the vegetables have direct contact with the pan.

Season the pan: Drizzle the vegetables with the remaining 2 tablespoons olive oil. Sprinkle them with the remaining 3/4 teaspoon salt and 1/4 teaspoon black pepper.

Set up for good browning: Turn the cabbage wedges so at least one cut side is flat against the sheet pan. Leave a few open spaces for the chicken you will add later.

Give the vegetables a head start: Roast the vegetables for 10 minutes. This gives the carrots and cabbage a head start so everything finishes at the same time.

Coat the chicken: While the vegetables roast, put the chicken thighs into the large bowl with the remaining mustard glaze. Turn the chicken in the bowl until all of the pieces are evenly coated.

Add the chicken to the pan: Pull the sheet pan from the oven and place it on the stove or a heatproof surface. Nestle the chicken thighs onto the pan between the vegetables.

Roast the full pan: Return the sheet pan to the oven and roast for 18 minutes. The chicken should start to brown at the edges and the onion should look softer.

Check and rotate: Rotate the sheet pan from front to back. If any cabbage wedges are browning too fast, turn them over with tongs.

Finish roasting: Roast for 7 to 10 minutes more, until the carrots are tender when pierced with a fork and the thickest part of the chicken is no longer pink in the center and reaches at least 165°F.