Cook the macaroni according to
the packet instructions, then drain
and set aside. Preheat the oven to
200°C/Fan 180°C/Gas 6.
Heat the butter in a large
saucepan over a medium-high heat
until the butter melts, then stir in
the flour using a wooden spoon
to make a very thick paste. Allow
the paste to cook for a minute until
browned slightly, then beat in a
ladleful of milk – it will get quickly
absorbed, so repeat. When you’ve
added all the milk, switch to a whisk
and add the stock, a little at a time,
whisking to avoid any lumps. As
soon as the liquids are incorporated,
add the cheeses, reserving a little
to sprinkle on top, along with the
mustard, sriracha and cayenne
pepper. Reduce the heat to a
simmer for 10 minutes, then season
to taste, remembering that when
you add the pasta the seasoning will
be diluted, so over-season.
Add the crabmeat, parsley and
macaroni to the sauce to combine
well and tip into a mediumsized
roasting dish. Sprinkle over
the reserved cheese and the
breadcrumbs and bake for 25–30
minutes until hot and bubbling.