1. Place potatoes in a large saucepan; cover with water. Bring to a boil, and cook 20 minutes or until very tender; drain. Cool, uncovered, 5 minutes. Return potatoes to pan, adding 2 1/2 tablespoons olive oil, 1/2 teaspoon salt, and pepper; mash to desired consistency. Keep warm.
2. Combine remaining salt, lemon zest, and rosemary in a small bowl; grind together with the back of a spoon until very well combined.
3. Preheat grill or grill pan.
4. Brush fillets with remaining 2 teaspoons olive oil; rub evenly on all sides with salt mixture.
5. Grill 2 minutes per side or until fish flakes easily with a fork. Serve immediately with lemon wedges and potatoes.