Heat the butter in a saucepan, stir in the flour and fill up with the fish stock. Simmer over a moderate heat for 8 minutes. Stir in tomato paste and add shrimp. Let simmer gently for another 5 minutes. Then puree well with the hand blender or mixer.
Heat the soup again and season well with salt, pepper and lemon juice. Fold in the whipped cream and flavor with the cognac. Serve immediately.