On the norte, in Santillana there is a restaurant called El pasaje de los nobles on the pedestrian area that serves a quality meal for about 10€. I had pote asturiano, tuna steak with watermelon and a decadent lemon mouse for desert, with house wine.
In San Vicente de la Barquera under the awnings where all the restaurants are, there is one with white and blue tile framinf the front door. It serves a great pote asturiano and grilled sardines.
In San Sebastian there are very famous pintxos places, A fuefo negro is one, the other is La cuchara de San Telmo. They are within 50 meters of each other but the entrance to La cuchara is on the back alley, by the church entrance on the plaza, not on the street itself.
Zarautz has a restaurant by a famous chef, Karlos Arguinano, facing the beach. He makes it a point to serve local produce at affordable prices. Also, after Zumai, if you take the traditional route and not the GR detour to see the flysh, in Elorriaga there is a little simple bar where the lady made me a beautiful grilled tuna steak and tomato salad and offered me a glass of txacoli from the vineyard from across the street.