Bring a pan of salted water to the boil and cook the macaroni according to the pack instructions until al dente. Drain well.
Meanwhile, heat the oil in a saucepan over a medium heat and gently fry the onion until it starts to soften, around 5 minutes. Add the garlic, chilli flakes, celery salt and mustard and cook for 1-2 minutes. Stir together then add the butter.
Once the butter melts, add the flour and stir until it all comes together. Slowly whisk in the milk, keeping it on a low heat and making sure to stir continuously so there are no lumps.
Slowly bring up the heat to thicken the sauce, then stir in three-quarters of the cheese and allow it to melt through the sauce.
Next, add the crab and drained macaroni, stir then transfer to one large or four individual ovenproof dishes. Sprinkle over the remaining cheese.
Preheat the grill to high. Place the dishes under the grill until the cheese has melted. Serve straightaway.