You will never find salmon and fennel red curry on the streets of Bangkok, I can tell you that. But if both ingredients had been widely available over there (and affordable to most people) decades ago, I'm pretty sure these two would have become a classic ingredient combination in red curry, much like chicken and eggplants or beef and bamboo shoots. Why? Because they go so surprisingly well together in Thai red curry. If you're skeptical, I don't blame you. I was, too, until I had the first bite.
The making of this curry is as uncomplicated as can be, and there's not much to talk about. The only thing I would like to stress is that you want to begin with a good curry base. And that means you want to fry your curry paste in coconut cream (derived from the first squeeze of grated mature meat which is the most concentrated) until the coconut fat "cracks" (splits). This is the traditional curry procedure. The herbs and spices in the paste get activated during this stage. In the end, the coconut fat will rise to the top of the curry, carrying with it the fragrance of the paste ingredients.
The problem is that many brands of canned coconut milk in the US market, both locally produced and imported, contain an emulsifier. And even if you're careful in scooping out the creamy part that rises to the top of the can, most of the time you can't get your coconut cream to "crack." If that happens, just do the best you can with what you've got. As long as you fry the paste in the coconut cream at the beginning, even though you can't get the coconut to render its oil, the end result will still be far better than if you were to dump the ingredients into a pot and boil them all together.