Heat a pan on medium fire, add butter, and let it melt, once melted quickly add the flour, cook for roughly 1 minute. Make sure the butter and flour mixture is not turned blonde or brown, as this is a white sauce the aim is to make a white roux (flour and butter mix)
Then slowly add the milk, while whisking the roux. Make sure to add the milk little by little so, lumps are not formed and you end up with a silky smooth mix. If by any chance there are lumps in the sauce, then refer to the article above as to how to rectify the sauce.
Leave the mixture on low heat, until the sauce is thickened, but make sure it does not thicken too much, so you will end up with a saucy mac and cheese after baked
Turn off the heat and add dijon mustard and mix well until incorpotrate. Then add all the shredded cheese, mix well, then pour in the cooked pasta to the same pot and mix well Transfer the whole mix into a baking tray
Heat oven to 180C conventional oven