Frank described the Spaniard as a player with huge potential, as he made him his seventh signing of the transfer window.
Fernandez made 22 league debuts keeping five clean sheets for second division side Huesca last season.
He has played for Spain at youth level and has one international cap, having been called up from the Under-21 squad to the full side in a friendly against Lithuania after some senior players withdrew under Covid-19 isolation rules.
Brentford impressed in their win over Arsenal in their first top-flight appearance since 1947 on Friday night.
Next up for the Bees is a London derby away to Crystal Palace.