Heat the oven to 180c/160c fan /gas mark 4. You will need a flat baking tray and 6 ramekins.
Put a flat baking tray into the oven to heat up. Brush each ramekin with Lurpak® butter making sure you do not miss any bits. Sprinkle over the Parmesan and coat the whole dish. Tap the dish upside down to remove any excess mixture. Set aside. Repeat with the other ramekins.
To prepare the soufflé, melt the Lurpak® butter in a medium sized saucepan over a medium heat. Stir in the flour and beat with a wooden spoon until smooth. Continue for a couple of minutes until you have a pale brown roux.
Lower the heat and slowly add the warm milk, whisking continuously until you have a smooth thick sauce. Continue to cook for a further 5 minutes. Add the cheese, mustard and tarragon and continue to cook for a further 2 minutes, stirring occasionally. Remove the pan and allow to cook for 5 minutes. Beat in the egg yolks one by one until you have a silky thick sauce. Cover with wet greaseproof and set aside.
In a separate, large and very clean bowl, add the egg whites and lemon juice. Beat until you reach a medium peak. Fold 2 tbsp of the egg whites into the mixture to loosen and then gently fold in the rest as carefully as you can so not to loose any volume. Spoon the mixture into the soufflé dishes. Filling each one to about 2/3 of the dish. Take a knife and run it around the mixture in the dish. Sprinkle each one with the remaining cheese and place into the oven on the pre-heat baking tray.
Cook for about 20 minutes. Serve immediately.