Grass-fed beef steaks sous vide.

A lot of the sous vide beef I do is grass fed and sous vide works just fine for it. I think with grass-fed beef there is just more variability
in the meat compared to normal US supermarket meat which is pretty uniform. My guess would be your typical Spanish beef, whether grass-fed or not, would be different than US supermarket meat. For instance, the beef I get from a local ranch tenderizes almost twice as fast as the supermarket beef I get.

In general, most sous vide cooking ranges should still work, you just need to fool around and see if your meat is more tender or more tough in general. If you cook some meat and it isn't as tender as you would like just bump up the cook time by a few hours and see if the extra tenderization helps out.

That's what's nice about sous vide, once you know what temperature you want the meat to be the only thing that matters is how long to cook it. The longer you cook, the more tender it gets.

I hope this allows you to get some great results, let us know how it does!