Creole Crawfish Etouffee

Name: Creole Crawfish Etouffee
Ingredients: 2 tablespoons unsalted butter; 2 tablespoons all-purpose flour; 1 stalk celery, diced; 1 onion, diced; 1 small green bell pepper, diced; 3 cloves garlic, minced; 3/4 cup chicken stock or reduced-fat low-sodium chicken broth; 1 tablespoon tomato paste; 1 pound shelled crawfish tails; 2 tablespoons chopped fresh parsley; 1/4 teaspoon cayenne pepper, or to taste; Coarse salt and freshly ground black pepper; Cooked rice, for serving; 2 scallions, white and pale green parts only, chopped