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Mix 50g butter and flour into a paste (known as beurre manié, see notes, below). Chill to firm for about 30 mins. Heat milk to boiling point with the onion, spices and bay leaf, then simmer gently for 5 mins. Remove from heat and allow to cool. Taste milk for flavour.
Strain the milk into a large pan, pressing down on the onion to extract the maximum of flavour. Place the pan back on the heat and gradually whisk in the butter paste, adding in small pieces until you have a thick sauce.
Season, then leave to cool for 3-4 mins. Whisk in egg yolks, grated gruyère and cheddar, plus two-thirds of the parmesan. Add the Worcestershire sauce and mustard. Check seasoning, leave to cool. Can be made 2 days ahead – bring to room temperature before continuing.
Brush the insides of a soufflé dish (approx 20 x 8cm deep) with the remaining 25g butter, using upward brush strokes. Coat the base and sides evenly with some parmesan and freshly ground black pepper. Chill to set. Cut the gruyère slice into diamond shapes.
Heat oven to 200C/fan 180C/gas 6. Coat a large, grease-free bowl with the lemon juice (see tip, below). Whisk the egg whites in the bowl, then whisk a third of them into the cheese mixture, to loosen. Carefully fold remaining egg whites into cheese mixture until well mixed, but still light.
Spoon half the mixture into prepared dish. Dot with brie and top with remaining mixture. Gently level the top. Arrange gruyère on top, scatter with remaining parmesan. Bake for 10 mins. Reduce oven to 180C/fan 160C/gas 4, bake for 15-20 mins. The soufflé should be evenly risen and slightly wobbly. Serve immediately – the centre will be soft, but will thicken slightly when served.