While the oven preheats, in a shallow dish mix together 4 cups room temperature water and 3 tablespoons kosher salt. Place the salmon in the water and wait for 15 minutes (this should be about the time it takes to preheat).
Pat the salmon dry with a clean towel. Rub it generously with oil and sprinkle it with the kosher salt and rosemary.
Bake the salmon skin side down for 5 minutes. Flip and cook another 2 minutes, until the internal temperature is 125 to 130 degrees. Allow to rest for a few minutes, then serve.