Favourite Cheese Sauce Recipe (vg, gf, oil-free option, nut-free option, customisable)

The other night I did my first ever cookery demo, and I loved it.  Probably mainly because I managed to crowbar in some cheese jokes. When I became vegan, I accepted the 'truth' that I would never enjoy mac n cheese again, never again make cheese jokes... How wrong I was. This is my favourite comfort …