Preheat oven to 400°F (200°C). Toss chicken breasts with olive oil, salt, pepper, and garlic powder. Bake for 20-25 minutes, or until cooked through. Shred with two forks. Alternatively, use leftover rotisserie chicken.

Toss broccoli florets with olive oil, salt, and pepper. Spread on a baking sheet and roast for 15-20 minutes, or until tender-crisp.

In a large saucepan, melt butter over medium heat. Whisk in flour and cook for 1-2 minutes, until a smooth paste forms (a roux). Gradually whisk in milk until smooth. Bring to a simmer, stirring constantly, until the sauce thickens. Reduce heat to low and stir in salt, pepper, nutmeg, and 3/4 cup of the cheddar cheese. Stir until cheese is melted and sauce is smooth.

In a large bowl, combine the shredded chicken, roasted broccoli, and cheese sauce. Stir well to combine.

Pour the mixture into a greased 9x13 inch baking dish.

In a small bowl, combine crushed Ritz crackers and melted butter. Sprinkle evenly over the casserole.

Bake for 20-25 minutes, or until the topping is golden brown and the casserole is bubbly.

Let cool for 5-10 minutes before serving.