Using a pastry brush, thoroughly grease a 25-30cm oval or round earthenware dish with a thin, even layer of butter. Coat with the breadcrumbs, shaking out the excess, then set the dish aside. Heat oven to 180C/160C fan/gas 4 and place a baking tray on the middle shelf to heat up.
To prepare the soufflé base, melt the butter in a small saucepan over a medium heat. Add the flour, whisk until smooth and cook to a nutty blond roux. Lower the heat, then gradually add the warm milk, little by little, whisking to keep the consistency smooth. Add the cheese and mustard, and continue to cook, stirring from time to time, for 3-5 mins. Remove from the heat and allow to cool a little. Add the egg yolks and stir until the mixture is silky and smooth. Season with white pepper (see tip) and keep warm. Can be made ahead up to this point (see tip).
In a large, clean, dry bowl, whisk the egg whites with the lemon juice to medium peaks.
Transfer the warm soufflé base to a large bowl and briefly whisk in a third of the whipped egg whites to lighten the base. Carefully fold in the remaining egg whites with a spatula or large metal spoon, delicately cutting and lifting the mixture to ensure minimum loss of volume and lightness. Taste and adjust the seasoning if necessary. Pour the soufflé mixture into the prepared dish so it is three-quarters full.
Slide the dish onto the hot baking tray and bake for 20 mins. Meanwhile, make the cheese sauce.
Bring the cream to the boil and add the cheese and some freshly ground white pepper, stirring continuously. Once the cheese has melted, remove from the heat and taste for seasoning. Add a dash of kirsch (if using), then pour the sauce into a warmed small jug.
Sprinkle the grated cheese over the soufflé and bake for a further 5 mins. Serve immediately, placing the soufflé and sauce in the middle of the table so everyone can help themselves.