Which means what exactly?
Examples:
Pasta alla carbonara
Pizza alla marinara
Trenette al pesto
In this particular use (for bolognese or pescatora, amatriciana, etc.) "a/al/alla" means "in the style of", BUT when it is pasta al ragù (al pesto, al pomodoro, etc., i.e. other main ingredients) the "a/al/alla" means "with".
And we have also our own exceptions, one says "Pappardelle sul cinghiale" (pappardelle is yet another type of pasta), literally "pappardelle on the hog", BUT you don't have the "a" in "spaghetti aglio e olio (e peperoncino)".
As a side note a "ragù" has nothing to do with the (usually horrible) tomato sauce that is used for "spaghetti bolognese" outside Italy.
Mai sentito parlare prima di pappardelle sul cinghiale, è un uso abbastanza dialettale mi sa.
Ovviamente non si può non concordare sul ragù. ;)
https://it.wikipedia.org/wiki/Pappardelle_sul_cinghiale
https://books.google.it/books?id=M_vYmUgN3e0C&pg=PA289&lpg=P...
you won't find, in any restaurant or "trattoria" in the area written on a menu "al" cinghiale.
It is typical - I believe - of "pappardelle", they are also sometimes (less common recipes) "sulla lepre" or "sul papero".