Step 1

Preheat oven to 350 degrees F. Combine broccoli, onion, olive oil, 1/8 tsp. pepper, and salt in a medium bowl; toss to coat. Mix together whole-grain mustard, Dijon mustard, rosemary (or thyme), and maple syrup (or honey), if using, in a small bowl.

Step 2

Pat chicken dry and season with the remaining 1/8 tsp. pepper. Heat canola oil in a 10-inch cast-iron or other ovenproof skillet over medium-high heat. Add the chicken and cook until the underside is nicely browned, 3 to 5 minutes. Remove the pan from the heat. Turn the chicken over and brush the mustard mixture over the top. Scatter the broccoli mixture around the chicken. Transfer the pan to the oven (see Tip).

Step 3

Roast the chicken and vegetables, stirring the vegetables once, until an instant-read thermometer inserted into the thickest part of the chicken registers 165 degrees F and the broccoli is tender and browned in spots, about 15 minutes. If either the chicken or vegetables are done before the other, remove them. Transfer the chicken to a clean cutting board and let rest for a few minutes. Transfer the broccoli mixture to a small bowl; keep warm.

Step 4

Meanwhile, place the skillet over medium-high heat and add vinegar and water. Bring to a simmer, stirring and scraping up any browned bits from the bottom of the pan. Simmer for 30 seconds. Remove from the heat. Add butter and whisk until melted.

Step 5

To serve: Slice the chicken 1/2 inch thick and divide between 2 plates, along with the vegetables. Drizzle the pan sauce over the chicken and vegetables.