Generously butter the inside of 6 ramekin dishes, ensuring you take the butter right up the sides of each dish. Place a small round disc of grease-proof paper on the base on each ramekin. Place the dishes on a baking tray and set aside.
Pour the milk into a medium sized saucepan and add the onion and bay leaf which will flavour the milk. Place over a low/medium heat and bring the milk to a gently simmer, take care not to boil the milk. Cook for minutes to allow the flavours to infuse the milk. Move from the heat and using a sieve strain the milk into a measuring jug and set aside.
To make the bechamel base sauce, melt the butter in a non-stick saucepan over a medium heat. Add the flour and stir well to combine to a roux. Gently cook the flour for 5 minutes to get rid of the raw taste of the flour, stirring continuously.
Gradually add 250ml of the warm milk into the roux and discard any excess milk. Once you have added the milk bring it to the boil and keep stirring to avoid it sticking.
Add the mustard and the cheese to the sauce, which should be thick and very smooth. Once the cheese has melted add the chives, a grating of nutmeg and season to taste with salt and pepper.
Place the sauce mixture into a large bowl and allow to cool for 5-10 minutes. Then beat in the egg yolks and mix until thoroughly combined. If you add the egg yolks when the sauce is too hot they will only cook in the sauce!
In a separate bowl whisk up the egg whites until you have stiff peaks. Add a large spoon of the egg whites into the cheese sauce and mix well to combine. Then add the remaining egg whites to the bowl and gently fold the egg whites through the cheese sauce. Take care not to beat the eggs, you want to keep the light airiness of the egg white, this will give you lighter soufflés.
Gently spoon the mixture evenly between the 6 ramekins. Place in a pre-heated oven at 200C for 18-20 minutes. Do not open the oven during cooking as this will only cause the soufflés to deflate. After 18-20 minutes the soufflés will have risen and should be golden on top.
Remove from the oven and place the ramekins onto a cooling rack. Leave the soufflés to cool completely.
Once cool, run a sharp knife around the sides of the ramekin. Gently turn the soufflés out onto your hand. Place the soufflés top side down on a clean baking sheet that has been lined with more grease-proof paper. The flat bottoms of the soufflés should no be facing upwards so remove the discs of paper from the bottom of each soufflé.
Mix together the Comte and Gruyere cheese for the topping and spread equally over each of the 6 soufflés. Pour a tablespoon of double cream over each soufflé.
Place the soufflés back into the oven for a further 10-12 minutes at 200C. Remove once the cheese has melted and has started to bubble. Serve immediately.
You can make these twice baked cheese soufflé ahead of time, up to the end of the first bake, Step 9 in the recipe card. After this first bake, remove from the ramekins and place top down on a baking tray lined with baking parchment. You want the flat bottom of the soufflés facing upwards. Cover the baking tray with clingfilm, taking care not to press down on the soufflés. Place in the fridge for up to 24 hours before serving, no longer as they contain egg. When ready to finish simply remove from the fridge and allow to come to room temperature. Then top the soufflés with the grated cheese and double cream and put them in for their second bake in the oven.These soufflés can also be frozen after the first bake, step 9 in the recipe card. Simply allow to cool and remove from the ramekin and place in a box suitable for the freezer and cover with a lid. When ready to cook, defrost thoroughly and complete the second bake on the soufflés.We have used Comte and Gruyere cheese in this recipe, however, Cheddar, Gouda, Fontina and Emmental would all work equally well. Simply choose a good melting cheese of your choice.We have used chives to flavour these soufflés but you could also add finely chopped parsley instead. Or if you would rather, leave the herbs out altogether.