Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Cook the rice according to package directions until slightly underdone. Set aside.

In a large skillet over medium heat, melt the butter. Add the diced onion and cook until soft, about 5 minutes. Stir in the minced garlic and cook for 1 minute more until fragrant.

Sprinkle the flour over the onions and whisk for 1 minute to form a roux.

Slowly pour in the chicken broth, whisking constantly until smooth. Then, slowly whisk in the milk. Bring the mixture to a simmer and cook for 3-5 minutes, stirring occasionally, until thickened.

Remove from heat and stir in 1 ½ cups of the shredded cheddar cheese until melted. Season with salt, pepper, and garlic powder.

Add the cooked rice, shredded chicken, and broccoli florets to the skillet and stir to combine everything evenly.

Pour the mixture into the prepared baking dish and top with the remaining ½ cup of cheese.

Bake for 25-30 minutes, until the casserole is hot and bubbly and the top is golden brown. Let it rest for 5-10 minutes before serving.