Strain any aromatics from your milk
Place a saucepan on the hob and melt the butter
Add the flour and, using the balloon whisk or a fork, mix the flour in
Splash in about 50ml of the milk and make a thick paste
Keep on adding around 50ml of milk, whisking until all the flour/butter mixture is combined
Bring gently to the boil and, once it’s popping gently, turn the heat down and stir occasionally for 5 minutes
Add in the cheese/cheeses and mustard (if using), and mix the half-cooked cauli and cheese sauce together.