Preheat the oven to 220°C/200°C Fan/Gas 7. Grease a 23cm (9in) round ovenproof dish with butter. It will need to be about 5cm (2in) deep.
Melt the butter in a saucepan over a medium heat. Add the flour and stir for 30 seconds. Slowly pour in the milk, whisking all the time until thick and bubbling. Remove from the heat and add the mustard and both cheeses. Season well with salt and freshly ground black pepper. Cool slightly before stirring in the egg yolks and parsley.
Place a frying pan over a high heat. Add the bacon and fry for a few minutes. Add the mushrooms and fry until all the liquid has evaporated and the bacon is starting to crisp.
Meanwhile, whisk the egg whites in a large clean bowl with an electric hand whisk until stiff and cloud-like. Add a good tablespoon of beaten egg white and beat well into the cheese mixture, then gently fold the remaining egg white into the cheese mixture.
Transfer the hot bacon and mushrooms into the base of the prepared dish. Spoon the soufflé over the top. Bake in the preheated oven for about 15–18 minutes, until well risen, golden and just set in the middle.
Serve at once, with dressed salad leaves.
Mary’s Tips: Not for freezing.
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