Heat the oven to 400°F. Put the spices in a spice or coffee grinder and pulse a few times, working in batches if necessary. (If you don’t have a spice grinder, put the spices in a sandwich bag, seal carefully, and whack with a rolling pin to crush the spices as best you can.) Put the salt in a large bowl, add the ground spices, and stir. Add the egg whites and mix thoroughly.
Line a large rimmed baking sheet with parchment. Put a layer of salt roughly the shape of the fish on the pan, then put the fish on top. Mound the rest of the salt on the fish, gently packing it onto the top and sides so the fish is completely enclosed and there are no gaps in the crust.
Bake undisturbed until you can smell the fish and the salt crust is hard and golden, 50 to 60 minutes. Bring to the table right away, and crack the crust with a serving spoon. Lift off the crust, then portion the fish and serve, passing the lemon wedges and aioli at the table.