Begin by cutting the chicken breasts into 1x1-inch cubes, ensuring even cooking throughout the casserole. Season the chicken pieces generously with salt and pepper to enhance flavor. In a heavy-bottomed pan, heat a splash of olive oil over medium-high heat, allowing it to shimmer. Brown the chicken in batches for about 5-7 minutes until golden on the outside, then transfer to a plate.

In the same pan, add a bit more olive oil if necessary, and toss in the finely chopped onion. Sauté the onion for about 5 minutes, or until it becomes translucent and fragrant. Next, stir in minced garlic, cooking for just 30 seconds until aromatic—be careful not to burn it.

Return the browned chicken to the pan, mixing it with the sautéed onions and garlic. Then, add the rinsed white long grain rice, ensuring it gets coated with the flavors in the pan. Pour in the chicken stock and water, stirring everything gently until combined.

Reduce the heat to a low simmer and cover the pan tightly. Cook the mixture for about 10 minutes without lifting the lid. After this time, carefully remove the lid and add in the broccoli florets. Stir gently and cover again, allowing it to cook for an additional 5-10 minutes.

Once the rice is cooked, preheat your oven to 425°F. Stir in the milk to increase the creaminess of the casserole. Next, fold in 1 cup of sharp cheddar cheese until melted and evenly distributed throughout the mixture.

Sprinkle the remaining cheddar cheese generously over the top of your Cheesy Broccoli Chicken and Rice Casserole. Place the pan in the preheated oven and bake for 10-15 minutes, or until the cheese is melted and bubbling with a golden crust.