How to Make It
Heat oven to 425° F, with the racks in the upper and lower thirds. Combine the butter and mustard in a small bowl. Place the chicken on a rimmed baking sheet and loosen the skin, using your fingers. Spread the mustard butter under the skin of each thigh, dividing evenly. Season with ¼ teaspoon each salt and pepper.
On a second rimmed baking sheet, toss the potatoes, Brussels sprouts, rosemary, lemon slices, oil, and ¼ teaspoon each salt and pepper.
Roast the chicken on the top rack until an instant-read thermometer inserted in the thickest part (avoiding the bone) registers 165° F, 20 to 27 minutes. Roast the vegetables on the bottom rack at the same time as the chicken, tossing once, until tender, 18 to 22 minutes.
Serve the chicken with the potatoes and Brussels sprouts on the side.