Gremolata Butter Baked Cod

Name: Gremolata Butter Baked Cod
Ingredients: 4 tablespoons (1/2 stick) unsalted butter; 4 (about 1-inch thick, 6-ounce) skinless cod fillets, thawed if frozen; 1 3/4 teaspoons kosher salt, divided; 1/2 teaspoon freshly ground black pepper; 12 ounces small Yukon Gold or red potatoes (1 1/2 to 2 inches wide, about 6); 2 medium shallots; 1/2 cup pitted kalamata olives (optional); 1 tablespoon olive oil; 3 cloves garlic; 10 sprigs fresh parsley, plus more for serving; 1 medium lemon