Cut the chicken breasts into approximately 1-inch cubes and season with salt and black pepper.
Heat the olive oil in a large oven-safe pan or Dutch oven over medium-high heat.
Once the oil is hot, add the chicken in a single layer.
Brown the chicken for 5–7 minutes, turning occasionally so several sides develop a golden crust. The chicken does not need to be completely cooked at this stage.
If necessary, brown the chicken in two batches to prevent overcrowding.
Transfer the browned chicken to a clean plate and set aside.
Reduce the heat to medium. Add the chopped onion to the same pan and cook for about 5 minutes, until softened and translucent.
Add the minced garlic and cook for approximately 30 seconds, stirring constantly.
Return the browned chicken to the pan.
Add the rinsed rice, chicken stock, and water. Stir everything together.
Bring the mixture to a boil, then reduce the heat to low and cover with a lid.
Cook for 10 minutes.
Add the chopped broccoli and stir gently to distribute it throughout the rice.
Cover again and continue cooking over low heat until the rice is tender and has absorbed most of the liquid. Stir occasionally to prevent sticking.
Meanwhile, preheat the oven to 425°F (220°C).
Once the rice is cooked, stir in the milk.
Add 1 cup of shredded cheddar and gently fold it into the Cheesy Broccoli Chicken and Rice Casserole until creamy and evenly combined.
Sprinkle the remaining 1 cup of cheddar evenly over the top.
Transfer the oven-safe pan to the preheated oven and bake for 10–15 minutes, until the cheese is completely melted and bubbling.
Let the casserole rest for about 5 minutes before serving.