Set the oven to 425F with a rack in the lower third and slide the empty rimmed sheet pan in while it heats. A hot pan sears the sweet potato on contact.

Cut the sweet potatoes into ¾-inch cubes, halve the brussels sprouts through the stem, and cut the red onion into 1-inch wedges. Cut sizes matter more than anything else here: the sweet potato goes in larger than feels right so it needs the same 45 minutes the chicken does.

Toss the vegetables in a large bowl with 2 tablespoons olive oil, the Dijon mustard, 1 teaspoon kosher salt and the black pepper until everything is coated.

Pat the chicken skin completely dry with paper towels. Rub the thighs with the remaining 1 tablespoon olive oil, then season with ½ teaspoon kosher salt, the garlic powder, crumbled rosemary and smoked paprika.

Pull the hot pan from the oven and spread the vegetables across it in one single layer with real gaps between the pieces, turning the sprouts cut side down. Do not crowd the pan, use two pans if you double the recipe.

Nestle the chicken thighs on top of the vegetables, skin side up and evenly spaced, so the rendering fat drips down through everything.

Roast 40 to 45 minutes. At the 25 minute mark, open the oven once and drag the vegetables from the hot outer edges toward the middle, leaving the chicken where it sits. Then leave it alone.

Check the thickest part of a thigh next to the bone for 175F. Thigh meat is better a little past the 165F safety mark and stays juicy.

Rest the pan 5 minutes, then squeeze the half lemon over everything and scatter the chopped parsley.

Scrape the vegetables back through the rendered fat and browned bits in the bottom of the pan, then serve straight from the pan.