How to Make It
Preheat oven to 375°F. Lightly spray a 13- x 9-inch baking dish with cooking spray.
Cook pasta in a large Dutch oven according to package directions for al dente. Drain pasta, and set aside. Do not wipe Dutch oven clean.
Melt butter in Dutch oven over medium-high, and add chicken, salt, and 1 teaspoon of the lemon pepper. Cook, stirring often, until chicken is beginning to get a little color but is not completely cooked, about 6 minutes. Add onion, celery, and 1 cup of the scallions; cook, stirring often, until vegetables are beginning to soften, about 4 minutes. Add flour, and cook, stirring constantly, 1 minute to blend completely. Gradually stir in milk and cream, gently scraping Dutch oven to release any flour mixture from bottom. Bring mixture to a boil over medium-high. Boil, stirring often, until sauce thickens, about 2 minutes. Remove from heat. Add cheeses, hot sauce, sour cream, cooked and drained pasta, and remaining 1/2 cup chopped scallions, stirring until well combined.
Spoon mixture into prepared baking dish; sprinkle with blue cheese. Top with panko, and sprinkle remaining 1/2 teaspoon lemon pepper over panko. Spray lightly with cooking spray.
Bake in preheated oven until pasta mixture is bubbly and panko topping is lightly browned, about 15 minutes. Garnish with additional chopped scallions.