Prepare the Chicken Salad Mixture

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish.
2. Combine Chicken and Veggies: In a large bowl, mix the shredded chicken with the diced celery, chopped red onion, and any other optional veggies or fruits (like grapes or apples) that you prefer.
3. Add Dressing Ingredients: Stir in the mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Taste and adjust the seasoning as needed.
4. Mix Until Combined: Make sure everything is well-mixed, and the chicken is evenly coated with the dressing.

Assemble the Casserole

1. Transfer to Baking Dish: Spoon the chicken salad mixture into the prepared baking dish and spread it out evenly.
2. Top with Cheese: Sprinkle the shredded cheese evenly over the chicken salad mixture.
3. Prepare the Cracker Topping: In a small bowl, combine the crushed crackers with melted butter. Stir until the crumbs are well-coated with butter.
4. Add the Cracker Crumbs: Sprinkle the buttered cracker crumbs over the cheese layer, covering the casserole evenly.

Bake the Casserole

1. Bake: Place the casserole in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and bubbly. The cheese should be melted and the cracker crumbs crispy.
2. Cool Slightly: Remove the casserole from the oven and let it sit for a few minutes before serving.