It's the N-621 from Potes to Riano.
I would also recommend taking the turn off onto the LS-243 up to Posada de Valdeon and on to meet up with the N-625 which runs from Riano up to Cangas de Onis.
Some fantastic views, and roads, and Posada is a good place for a lunch or coffee break. Whereas Riano has nothing to offer, being a bit of a depressing town rebuilt in a ne location when the reservoir flooded the original town