Place the chicken, breast side up, in a roasting pan or ceramic baking dish just large enough to hold the chicken and radishes, and sprinkle the cavity liberally with salt and pepper. Put the lemon and thyme in the cavity. Tie the legs together with kitchen string and tuck the wings under the body. Cut any larger radishes in half so they are all about the same size and place around the chicken.
Pat the chicken dry with paper towels. Brush the chicken and radishes all over with the melted butter and sprinkle them both liberally with salt and pepper.
Roast for 1 hour and 15 minutes, until the juices run clear when you cut between the leg and thigh. Cover the pan or baking dish with aluminum foil and allow to sit at room temperature for 15 minutes. Carve the chicken and serve with the radishes and pan juices.