Combine the sugar, salt, black pepper and rosemary in a small bowl. Lay out a large piece of plastic wrap on a baking sheet–this piece needs to be large enough to wrap up your piece of salmon.
Spread half the mixture, in the size and shape of your salmon fillet, on the plastic wrap. Lay your salmon fillet, skin side down on top of the mixture. Top the other half with the remaining mixture, spreading evenly over fish. Fold up the edges and wrap tightly.
Place another baking sheet on top of the salmon with some heavy cans to weigh down the salmon slightly. Refrigerate for 4-6 hours.
After your fish is cured, you’ll need to let it dry out overnight to create the pellicle. Unwrap fish carefully, you’ll notice that there is a lot of extra moisture in the plastic wrap. Completely rinse the dry brine mixture off the salmon and pat dry.
Place salmon on a wire rack on your baking sheet, and dry out overnight (about 8-12 hours) in the fridge.
Preheat your pellet grill to 225F. Once preheated, place salmon, skin-side down on grill. Cover and cook for about 90 minutes, or until internal temperature reaches 140F (it will continue cooking slightly when you remove it from the heat).