Remove giblets, rinse chicken, and dry thoroughly with paper towels. Place in heavy-duty roasting pan. Insert garlic cloves, thyme sprigs, and lemon into cavity. Bring chicken legs forward and tie together. Spread softened butter over entire surface of chicken; sprinkle with salt and pepper. Roast at 425 degres until skin is deep golden brown and crisp and juices run clear (about 1-1/2 hours). Insert instant-read thermometer into the breast, then the thigh. Breast temperature should read 180 degrees, and the thigh, 190 degrees. Roast at 425 degrees until skin is deep golden brown and juices run clear (about 1-1/2 hours).
Remove chicken from oven and let stand 10 to 15 minutes so the juices settle. Meanwhile, pour the pan drippings into a shallow bowl. Leave any brown baked-on bits in the bottom of the roasting pan. Pour the remaining drippings and the juices that have collected under the resting chicken back into the roasting pan. Place on stove over medium high heat and cook about a minute. Add flour and whisk. Add chicken stock and cook until reduced (about 4 minutes). At the end, add cube of butter and whisk until melted.