Does this recipe approximate what you had?
I found it by searching on 'catalan fish and potatoes'
The rough idea is to cook the potatoes (sliced) and onion (and tomato) till partially tender. Top with the fish, and pour over seasoning, and bake.
I did something like that recently with a recipe from one of my Spanish cookbooks. In that case the seasoning was sherry vinegar and oil (and salt, paprika etc). It was good except for the fact that my fish fillets were mislabeled arrowtooth flounder, which becomes horribly mushy when cooked. The plus side was that baked like this, it was easy to remove the offending fish pieces, and enjoy the potatoes.