You don't need to let it stand as long as you've mixed well after half the milk has gone in, like this:
Melt half an oz of butter/marge in a jug.
Add an egg, 4oz plain flour, pinch of salt and 1/4pt milk. Mix well. Then mix in another 1/4pt of milk.
I sometimes replace the milk with veg or chicken stock (omit the salt) and it's still yummy, just a bit more savoury. And actually I once forgot the egg and they were still fine!