Step 1: Prepare the Ingredients

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch casserole dish with butter or non-stick spray.
2. Cook the pasta according to the package instructions until it’s just al dente. Drain and set aside.
3. If using fresh broccoli, steam it for about 5 minutes until it’s tender but still bright green. If using frozen, blanch in boiling water for 2-3 minutes. Drain and set aside.
4. In a large skillet, heat 2 tablespoons of olive oil and 1 tablespoon of butter over medium heat. Sauté the chopped onion and minced garlic for 3-4 minutes until they become translucent and fragrant.

Step 2: Make the White Cheddar Sauce

1. In a medium saucepan, melt 3 tablespoons of butter over medium heat.
2. Whisk in the flour to form a roux. Cook for 1-2 minutes, stirring constantly, until it becomes a light golden color.
3. Gradually whisk in the milk, ensuring there are no lumps. Continue cooking and stirring until the mixture begins to thicken, about 5-6 minutes.
4. Lower the heat and add the shredded white cheddar cheese, Dijon mustard, ground nutmeg, salt, and pepper. Stir until the cheese is fully melted and the sauce is smooth. Remove from heat.

Step 3: Assemble the Casserole

1. In a large bowl, combine the cooked chicken, steamed broccoli, cooked pasta, sautéed onion, and minced garlic. Mix well.
2. Pour the creamy white cheddar sauce over the mixture, and gently fold until everything is well-coated.
3. Transfer the mixture to the greased casserole dish. Smooth out the top with a spatula.

Step 4: Add the Topping

1. Sprinkle the shredded mozzarella cheese evenly over the top of the casserole.
2. If you like a crunchy topping, mix 1 cup of breadcrumbs with 2 tablespoons of melted butter and ½ cup of Parmesan cheese. Sprinkle this mixture evenly on top of the mozzarella layer.

Step 5: Bake the Casserole

1. Place the casserole dish in the preheated oven. Bake for 25-30 minutes or until the top is golden and bubbly.
2. For an extra crispy top, turn the oven to broil for the last 2-3 minutes. Keep a close eye to prevent burning.
3. Remove the casserole from the oven and let it cool for 5 minutes before serving.