1. Heat oven to 475 F. Place potatoes and half the butter in a roasting pan. Roast, tossing once, until potatoes are golden, 20 to 25 minutes.
2. Place chicken, on top of potatoes; season with salt and pepper. Roast until chicken begins to brown, about 20 minutes.
3. Scatter asparagus, lemon, remaining butter and thyme around chicken. Roast until asparagus is tender and chicken is opaque throughout, 5 to 15 minutes. Serve chicken, vegetables, and lemon drizzled with pan juices.