Preheat the oven to 350F.

Place the celery root in a large roasting pan, and add the olive oil and a generous amount of salt and pepper. Turn to coat.

Prepare the sage butter by combining softened butter with finely chopped fresh sage. Carefully lift up the skin from each chicken thigh and place a spoonful of the butter under.

Replace the skin, and repeat with the remaining pieces of chicken. Season the skin with salt and pepper, then place them on top of the celery root.

Roast for 40-45 minutes or until the chicken has reached an internal temperature of 165F.

While the chicken is cooking, add some oil to a skillet and place over a medium -high heat. Add the sage leaves, and remove them to a paper towel when they have turned crispy.

Serve the fried sage leaves over the chicken.