For the Curried Apple and Fennel:
Slice the apples and the fennel into thin strips. In a skillet, toast the curry powder and add grapeseed oil to cover. Simmer for 5 minutes and strain through coffee filter. Place apples and fennel in a container with curry oil. Cryovac the mixture and reserve.
For the Pickled Onions:
Slice onions into thin strips. Combine sugar and vinegar, and add salt to taste. Place onions in container with the pickling liquid. Cryovac the mixture and reserve.
For the Pickled Mustard Seeds:
Place the mustard seeds and apple cider vinegar in a pressure cooker and cook under full pressure for 20 minutes. While the mixture is hot, stir in the sugar. Strain and reserve.
For the Dill Crème Fraiche:
Place dill in Vita-Prep, add liquid nitrogen, and blend until dill is completely pulverized. Strain liquid nitrogen and dill through a coffee filter-lined fine chinois. Empty the dill powder into chinois and dust into the creme fraiche. Fold the mixture and season.
For the Mokume-Gane Fish:
Par-freeze the salmon and halibut filets on parchment paper. Combine sugar and salt. Slice frozen halibut into 1-2mm-thick slices. Place the large slices on cooking spray-greased parchment paper. Continue slicing, making sure to to place a greased piece of parchment paper between each layer of fish. Place in fridge to thaw. Repeat procedure with frozen salmon, making sure that salmon slices are thinner than halibut slices. Place in fridge to thaw.
Heat the rubber mold in an oven until soft. Cut potato into rough slices, placing each curved end at random areas on the mold. Fit the second mold over the first and Cryovac. Remove from the plastic. Line a rubber mold piece with plastic wrap. Place two pieces of salmon onto the mold. Lightly dust the entire top of the fish layer with transglutaminase. Heavily dust the fish with the sugar and salt mixture. Alternately layer the halibut onto the salmon, making sure to keep the grain identical. Repeat the dustings with transglutaminase and curing mixture.
Repeat alternate layering until fish is 2 inches-thick. Cover with plastic wrap and top part of mold.Vacuum the the fish and mold. Transfer to refirgerator for 30 minutes. Gently remove bag without disturbing the fish layers. Refrigerate for an additional 4 hours. Freeze the fish until completely frozen through. Remove mold and plastic wrap. Trim edges into a perfect rectangle. Slice the fish thinly and reserve.
To Assemble and Serve:
Toast the pumpernickel bread. Place a layer of fish on the plate. Arrange dill crème fraîche, mustard seeds, pickled onions, celery leaves, and thin slices of pumpernickel.