Yes, lentil soup is available, and frequently delicious. However, each lentil soup I have had on any Camino, typically has tiny bits of chorizo sausage floating in it. Just be aware if it bothers you.
Incidentally, the sausage DOES impart a delicious flavor. I am not vegetarian. Actually I HATE vegetables, I have done so my entire life, but that is my bad.
But I do appreciate plain food and home-type cooking done well. Across Spain and Portugal, the lentil soup or Sopa de Abuela (grandmother's soup) are typically the best. But if you eschew meat of all types, it is caveat emptor...
Hope this helps.