Cajun Shrimp Alfredo

Name: Cajun Shrimp Alfredo
Ingredients: 16 ounces uncooked fettuccine pasta; 1 pound medium peeled, deveined raw shrimp; 2 teaspoons Cajun seasoning; 2 tablespoons olive oil; 1 cup finely chopped yellow onion (from 1 onion); 1/2 cup finely chopped celery (from 1 stalk); 2 garlic cloves , minced; 1/3 cup chicken stock; 1 cup heavy cream; 4 ounces Parmesan cheese, grated (about 1 cup); 1 teaspoon coarsely ground black pepper; 1 teaspoon kosher salt; 1/4 teaspoon cayenne pepper; 1/3 cup sliced scallions , for garnish