If you can get your hands on fresh monkfish for this, this should obviously be your first choice. But here in Germany fresh monkfish is hard to get and it's ridiculously expensive. But since it freezes really well the frozen kind will do nicely in this stew.
I used parsley in this recipe, but you could aswell use cilantro, which a lot of portuguese recipes call for, but I simply loathe cilantro :D