A poorly chosen example, Eric, I'm afraid, since "lasagna" is also widely used for this dish in italian... See for instance :

www.incucina.tv/ricette/1...icetta.asp


and many, many other instances.

The point is that, contrary to "spaghetti", where you get a thousand of the things in your plate, "lasagna" is cooked as a single dish, a unique "pasta", which you can then cut in a number of portions... Of course, since there are many ways of doing "lasagna", you will find mention of "lasagne" . And you can have individual lasagne too :

it.primopiatto.barilla.co...c64_IT.htm


So, anyway, you can say both.