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Preheat the oven to fan 170C/conventional 190C/gas 5. Place a baking sheet on a centre shelf. Break or chop the spaghetti into finger lengths. If using uncooked spaghetti, boil it in a saucepan of salted water until quite soft, for about 7-10 minutes, then drain.
Melt the butter in a medium saucepan and remove a tablespoonful to a 1¼ litre ovenproof soufflé dish. Brush the melted butter well around the base and sides. Add the breadcrumbs and shake around to coat thoroughly.
Heat the milk to boiling in another pan and set aside. Stir the flour and mustard powder into the pan with the remaining butter and heat for a minute, stirring, until sandy coloured. Gradually beat in the hot milk until you have a thick, smooth sauce. Remove from the heat.
Stir in all but 2 tbsp of the cheddar, all the spring onion and the shredded leaves. Stir the spaghetti into the sauce and mix in the egg yolks (you can prepare to this stage up to a day ahead).
Place the 4 egg whites in a large grease-free bowl with a pinch of salt. Whisk until you have a firm foam with floppy peaks. Beat a quarter of this into the spaghetti mixture to loosen it, tip into a large bowl and fold in the rest.
Spoon the mixture into the prepared dish and level the top. Run your thumb around the edge of the dish to make a “gully” and sprinkle over the last of the cheese. Put the soufflé on the hot tray in the oven and bake for 30-35 minutes until it has risen and is golden. Serve immediately.