A really simple bechamel-type sauce to boost pastas, cover vegetables, bind 'fish' pies... it's so versatile and not fatty at all. Full fat or semi-skimmed milk can be used as the base and it can be flavoured with cheese or herbs as you will.
Simply whisk 1-1.5 tblsp plain flour (thin sauce) or up to 3 tblsp plain flour (thick sauce) per pint of milk while the milk is cold until you see no lumps. Add herbs and seasoning. Then gently bring the sauce to boil while stirring constantly. Turn the heat down and simmer gently for 3-5mins, still stirring occasionally. Any cheese goes in at the end while it's still hot.
And that's that. I like really thick sauce, so I tend to use 2.5 spoons of flour.
I personally like to add a bit of strong cheese and herbs de provence or sage, depending what I'm using it with.