Core and coarsely chop apples; process in a blender or food processor until liquefied. Strain through a fine sieve, reserving juice and discarding solids. Place juice in a small saucepan. Bring to a boil, reduce heat, and cook until liquid is reduced by half. Stir in vinegar and cream. Simmer 2 minutes, remove from heat, and set aside.
Remove stems from kale, and cut into 2-inch pieces. Cook in boiling salted water 5 minutes; drain. Plunge in ice water immediately to stop the cooking process, drain, and set aside.
Sauté celery root in 1 tablespoon melted butter over medium heat 5 minutes or until tender, and set aside.
Melt 1 tablespoon butter; add shallot and bacon. When bacon is almost crisp, add kale, and cook until thoroughly heated. Set aside.
Score fish skin diagonally two or three times, and sprinkle with salt and pepper. Melt remaining 1 tablespoon butter in a cast iron skillet. Sear fish, skin-side up, until lightly browned. Press fillets flat with a metal spatula to prevent curling. Transfer skillet to oven, and bake at 350° for 5 to 7 minutes or until fish flakes easily with a fork.
Arrange kale in the center of the plate; sprinkle celery root around kale. Top with fish, and spoon sauce around celery root.