This was something of a legend in its own lunchtime when I was growing up, namely because it's the only thing my dad ever cooked...
For 3/4 I tend to use 2 cans of tuna and a lot more cheese than they say. Also I don't use the breadcrumbs as Mom says that it dries the sauce too much.
Talking of which don't make the sauce too thick, the cheese and then the baking will do that anyway. Sorry meant to say that I do the milk and onions bit of the sauce, ie before the thickening, so that it can stand for a long time to absorb the flavours before straining it.