Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Heat olive oil in a large skillet over medium-high heat. Add chicken and cook until browned on all sides.
Add onion to the skillet and cook until softened. Stir in broccoli florets and cook for a few more minutes.
In a large bowl, combine cooked chicken, vegetables, rice, chicken broth, cream of mushroom soup, milk, salt, and pepper.
Pour the mixture into the prepared baking dish. Top with shredded cheddar cheese. Bake for 45-50 minutes, or until the casserole is bubbly and the rice is tender.
