Begin with the potato salad. Put the potatoes, unpeeled, into a large saucepan of cold, salted water. Bring to a boil and cook for 12-15 minutes until tender. Drain, and then add back to the pot.
In a large sauté pan cook bacon until crispy. Remove with a slotted spoon and drain on a paper towel then cut into squares. Drain some of the fat if there is too much (leave about 2 tablespoons) then add the onions to the pan. Cook over medium heat stirring the onions until caramelized. Add the bacon and onions back to the pot of potatoes when done. Smash the potatoes up with a wooden spoon or large fork. Set aside.
Make a vinaigrette by combining the cider vinegar with oil and Dijon mustard. Whisk until combined. Pour this over the warm potatoes and stir to combine smashing up the potatoes with a wooden spoon. Season with salt and pepper then fold in the chopped green onions and chopped parsley.
Now prepare the sauce. In a medium saucepan add the peeled celery to the milk and chicken stock. Bring it up to a simmer and cook the celery until tender about 5 minutes - do not overcook it otherwise it will go brown. Pour the mixture into a blender, drizzle in some extra-virgin olive oil and puree. Add back to the pot and reduce until you have a consistency that coats the back of a wooden spoon. Season to taste.
Season the cod fillets with salt and pepper. Heat a 2-count of olive oil over high heat and add the butter. Place the cod fillets into the pan and cook for 5 minutes on one side until crispy and golden. Turn the fillets over and finish cooking for 1 more minute.
To serve, place a heap of warm German salad on the center of the plate. Place the crispy cod filet on top and then spoon the celery sauce over and around the plate.