How to Make It
Preheat oven to 350°F. Microwave 1 tablespoon of the butter in a small microwavable bowl on HIGH until melted, about 30 seconds. Add crumbled crackers; toss until coated, and set aside.
Melt remaining 3 tablespoons butter in a large saucepan or Dutch oven over medium. Add flour, mustard, and garlic; cook, whisking constantly, until flour is toasted (be careful not to burn), 1 to 2 minutes. Slowly whisk in stock and half-and-half; cook, whisking constantly, until thickened, about 5 minutes. Whisk in salt and pepper; remove from heat. Add broccoli, rice, chicken, and cheese; fold to combine.
Spoon mixture into an 11- x 7-inch baking dish coated with cooking spray. Sprinkle evenly with cracker mixture. Bake in preheated oven until golden brown and bubbly around the edges, 30 to 35 minutes. Sprinkle evenly with scallions. Serve.