Pre-heat the oven to 450 degrees on convection if your oven has it.
Place the diced sweet potatoes at the bottom of a roasting pan, and mix with a drizzle of olive oil to prevent sticking.
Rub the butter all over the chicken, including under the skin and inside the cavity, along with the thyme, kosher salt & pepper.
Place the lemon halves and bay leaf inside the cavity of the chicken.
Tie the legs of the chicken together with kitchen twine.
Place the chicken on top of the sweet potatoes in the roasting pan and roast for 30 minutes at 450 degrees.
Reduce the oven temperature to 375 degrees and rotate the roasting pan promote even browning. Continue roasting for about 20 to 30 more minutes, or until the internal temperature of the thickest part of the thigh reads 165 degrees.
Transfer the chicken to a cutting board and let rest for 15 minutes.
Use a slotted spoon to transfer the sweet potatoes to a serving dish.
Carve the chicken and serve with the roasted sweet potatoes and a drizzle of the pan juices.
Roasting times vary based on the size of your chicken and the type of oven you are using.