How to Make It
In a medium skillet, melt the butter with the vinegar. Add the leeks, onion and garlic and season with salt and pepper. Cook over moderate heat until the leek and onion have softened, about 5 minutes. Stir in the flour, then stir in the stock and simmer until slightly thickened, about 2 minutes. Transfer the leek mixture to a blender and puree until smooth. Return the puree to the skillet and stir in the cream. Simmer over low heat until the sauce thickens, about 5 minutes. Season with salt and pepper.
In a large nonstick skillet, heat the olive oil. Season the salmon with salt and pepper and cook over moderately high heat until light pink throughout, about 4 minutes per side. Transfer the salmon to plates and top with the leek fondue. Garnish with the parsley and serve immediately.