Preheat oven to 475 degrees. Place potatoes and half the butter in a roasting pan; season with salt and pepper. Roast, tossing once, until potatoes are golden, 20 to 25 minutes.
Place chicken, skin side up, on top of potatoes; season with salt and pepper. Roast until chicken begins to brown, about 20 minutes.
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.
We like dark, crispy skin; to achieve this, use a shallow roasting pan (that means three inches deep or less). Deeper pans trap more steam, which keeps meats from browning well.