Preheat oven to 400 degrees F.
Saute walleye over high heat until golden brown and flip. Place in the oven for 4 to 6 minutes or until fish is cooked to medium (about 140 degrees F).
Boil diced celery root and Granny Smith apples in salted water until tender and remove from liquid. Add butter and mash with potato masher until smooth. Season with salt and pepper.
Toss julienned apples with lemon oil and watercress and set aside. Place a small amount of puree on each plate and cover with a piece of fish. Garnish with apple salad.