Creole Crawfish Pie

Name: Creole Crawfish Pie
Ingredients: 2 (9-inch) refrigerated piecrusts (preferably homemade); 1/4 cup unsalted butter (plus 2 tablespoons, if needed); 3 tablespoons all purpose flour , plus more for work surface; 1 large (12-ounce) white onion , chopped (about 2 cups); 1 large (12-ounce) green bell pepper , chopped (about 1 1/4 cups); 1 medium (1 1/2-ounce) celery stalk , chopped (about 1/3 cup); 3 garlic cloves , minced (about 1 tablespoon); 1 (16-ounce) package frozen peeled Louisiana crawfish tails , thawed in package and undrained; 2/3 cup condensed cream of mushroom soup (from 1 [10.5-ounce] can); 1 1/2 tablespoons granulated sugar; 2 teaspoons Creole or Cajun seasoning , plus more to taste; 1 teaspoon onion powder; 1 teaspoon chopped fresh basil, plus more to taste; 1 teaspoon chopped fresh rosemary , plus more to taste; 1/2 teaspoon garlic powder; 1 large egg, beaten; 1 tablespoon water; Thinly sliced scallions, for garnish