For the stock, heat the butter and vegetable oil in a large pan and gently fry the leeks, onions and fennel for 3-4 minutes, or until softened.
Add the white wine to the pan and bring to the boil. Add the smoked haddock, pour in the water and bring back to the boil. Reduce the heat and simmer for 30 minutes, skimming off any scum that rises to the surface, until the haddock is cooked through.
Strain the haddock, reserving the cooking stock. Chop the haddock into bite-sized pieces.
For the soup, heat the butter with the vegetable oil and fry the leeks, shallots and garlic for 3-4 minutes, or until softened. Add the potatoes and the chopped smoked haddock to the pan.
Add the reserved cooking stock and bring to the boil, then reduce the heat and simmer for 10-15 minutes, or until the potatoes are tender. Stir in the cream and briefly blend the soup with a stick blender.
Serve the soup with crusty bread and sprinkle with the chopped fresh parsley and a sprinkling of freshly grated nutmeg.