Make the stock: Place the White fish bones and heads in a large pot with cold Water. Add a halved Onion, a few chunks of Carrot, the green tops of the Leek (green tops), Celery stalks, a couple of Bay leaves, a teaspoon of Black peppercorns and a handful of Parsley stems. Bring to a gentle simmer, skim any foam, and cook uncovered 30–40 minutes.
Strain the pot through a fine sieve, discarding solids, and reserve the liquid as your Fish stock.
Prepare the vegetables: slice the Leek (white and light green) and dice the remaining Carrot, Celery root (celeriac) and Waxy potatoes into bite-sized pieces.
In a heavy soup pot, melt the Butter over medium heat. Add the leek, diced carrot, celeriac and potatoes and sweat gently until softened, about 5–7 minutes, without browning.
Sprinkle in the All-purpose flour and stir to coat the vegetables; cook 1–2 minutes to remove the raw flour taste. Deglaze with the Dry white wine, scraping any browned bits.
Slowly add about 1–1.5 liters (4–6 cups) of the warm Fish stock, stirring to combine. Bring to a simmer and cook until the potatoes are tender, about 10–12 minutes.
Stir in the Heavy cream to enrich the soup, then season gently with ground White pepper, Sea salt and a small pinch of Sugar to balance the flavors. Keep the soup at a gentle simmer—do not boil hard once the cream is added.
Cut the Cod fillet into large chunks and add to the simmering soup; poach until just opaque, 3–5 minutes. Add the Cooked shrimp at the end to warm through for 1–2 minutes.
Remove and discard any remaining Bay leaves. Finish the soup off the heat with plenty of chopped Dill and Chives and a bright squeeze of Lemon. Taste and adjust seasoning before serving.
Ladle into bowls and serve hot—this soup is lovely with crusty bread to soak up the silky broth.
