Preheat broiler and set oven rack to about 6 inches below broiler element.
In a small bowl, stir together mayonnaise, harissa, lime zest and juice, and coriander seeds. Season with salt and pepper; feel free to adjust flavor and heat level by adding more harissa, if desired.
Line a rimmed baking sheet with aluminum foil. Lightly season salmon all over with salt and pepper. Set salmon on prepared baking sheet and rub a thin, even layer of harissa mayo all over the surface and sides.
Broil salmon until browned on top and the center registers 115 to 125°F (46 to 52°C) for medium-rare to medium, respectively, about 5 minutes.
If salmon becomes well browned on top before it is cooked enough in the center, switch off the broiler and set the oven to 425°F (220°C), then continue cooking until done (this shouldn't take more than 1 to 2 minutes longer).
Transfer salmon to plates or a platter and serve.
