Preheat oven to 425°. Place 6 lemon slices and sage leaves under skin of chicken. Put remaining lemon into cavity. Tie legs together with twine, and tuck wings under. Brush 1 teaspoon oil over chicken. Place chicken in roasting pan; roast in lower third of oven for 1 hour 15 minutes or until an instant-read thermometer registers 165°. Transfer chicken to a cutting board; let rest for 15 minutes.
Meanwhile, cut root vegetables into matchsticks. Toss with potatoes in a baking pan with remaining oil and thyme. Roast, stirring occasionally, for 45 minutes or until tender.
Remove skin from chicken. Discard lemons from cavity. Slice enough chicken to serve 4 (such as breasts), and serve with half of vegetables.