In a heavy-based saucepan reduce the fish stock to 300ml. In a separate saucepan reduce the combined alcohols to 400ml. In a separate saucepan reduce the cream to 750ml. In a heavy-based saucepan melt the butter until foaming. Add the shallots, onion, fennel, garlic and thyme and sweat for 3 minutes without colour. Add the reduced alcohol and fish stock and bring to the boil. Add the cream and bring back to the boil. Reduce the heat and cook for 10 minutes. Remove from the heat and pass the fish veloute through a fine chinois. Season to taste with salt and freshly ground pepper and a squeeze of fresh lemon.