Preheat the oven to 200 degrees Celsius. Peel the celeriac and cut it into chunks. Toss the chunks with olive oil, thyme, salt and pepper and roast until tender, about twenty-five minutes.
Meanwhile, peel the apples, cut them into pieces and cook gently with a splash of water and a pinch of cinnamon. Puree into unsweetened applesauce.
Season the pork tenderloin with salt and pepper. Sear it all over in a hot pan and then finish cooking briefly in the oven until pink in the center. Let the meat rest.
Slice the pork tenderloin, arrange the celeriac on a plate and serve with a spoonful of warm applesauce.