Is spaghetti and bechamel sauce an authentic Italian dish?

In Italy, besciamella is usually only used on pasta when it's going to be cooked in the oven: pasta al gratin, pasta al forno, or as part of lasagne.

As for your question - is it authentic to just toss some pasta with it? I would use the same method I'd use to answer the infamous "is X a sandwich?" questions: if somebody asked for a plate of spaghetti at a restaurant and they came covered in besciamella, I think basically every Italian would be baffled. I've never seen it on a menu, never seen it on a table at anyone's place and never heard it discussed until I read this discussion, which makes me inclined to say it's not authentic. It could still be delicious though!

I use it to make a slightly less rich 'carbonara-esque' sauce with pasta; bacon [or pancetta] onion, garlic. Make a roux round that, then milk, mushrooms, cheese, done.

I always considered it just a poor-man's carbonara - though it's fabulously tasty.

However, though it seems to have fallen in popularity under its original name, salsa colla, Béchamel actually originated in Tuscany & was later taken to France & renamed.

I'd say that gives us every moral right to put it on pasta & call it 'authentic Italian'.

Traditional (authentic?) Lasagna is made with Bechamel sauce.

Anyway, what does authentic mean, and how long a food tradition makes something authentic.

For example, Pasta Grannies

I think the ground principle behind the use of bechamel can be worded by saying that it's used to keep things together (and wet!) and to have a surface that becomes crunchy and brown when you bake pasta (or other things, for instance, vegetables)