Thinking under-seasoned means it needs more salt. It may, but it may also need an acid or a base, or something bitter, or sweet, or some umami to balance it out. When I need to adjust seasoning I like to start with the acid since that's the easiest to correct if you're wrong, and then go from there.
This recipe is fairly simple, but it does require you to use your judgment when adjusting the balance of flavors among the wine's acidity, the butter, and the salt you season with at the end. Not to mention the heat from the harissa, depending on the brand you use. Harissa adds a layer of nuance, offering added body and warmth from spices such as caraway and cumin.
"It is always good to add zest to your fish when frying, baking, grilling or poaching after it is cooked," Darkwah recommends. "[This] brings out the umami flavor of the fish." Like juice, zest works to balance and highlight other qualities: brightness versus richness and umami. It's the unexpected ingredient that makes a pasta dish sing and that elevates just about any meat or vegetable dish or dessert with a fragrant lift.