To a small saucepan, add the apple cider and bring it to a light boil. Reduce the heat to a simmer, stirring often with a wooden spoon. After about 25 minutes, the cider will reduce and become thick, like a syrup. Turn off the heat and immediately pour the cider glaze into a bowl to allow it to cool.
Meanwhile, arrange the salmon, skin-side-down, on a parchment-lined baking sheet. Drizzle the olive oil all over the top of the salmon and season with salt and pepper. Sprinkle the chopped rosemary all over the tops of the salmon. Roast until firm, about 15-20 minutes (ideally, salmon should be cooked on the outside while still moist on the inside).
Transfer salmon to serving plates and drizzle the cider glaze all over the top of the salmon.