Preheat oven to 375 degrees and spray a baking dish with cooking spray to prevent sticking. (Use foil for easy cleanup.) Cut filets into individual serving sizes if large, about 3 inches wide. Sprinkle with kosher salt and rinse with water. Pat dry and place on prepared baking sheet. Squeeze fresh lemon juice over filets. Sprinkle with kosher salt and cracked pepper. Chop fresh rosemary finely and sprinkle on top. Spread ranch dressing evenly on top. Bake for 10 minutes, skin side down (if yours have skin). Transfer to top rack and broil for 1-3 minutes or until top mixture is hot and bubbly and beginning to brown. You just want little specks of brown starting to happen. Serve immediately.