Bring the rice and water to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the rice is tender, and the liquid has been absorbed, 20 to 25 minutes.
Preheat an oven to 350 degrees F (175 degrees C).
Heat a large skillet over medium-high heat and stir in the ground beef. Cook and stir until the beef is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Add chopped onion to beef and cook and stir for 3 to 5 minutes. Stir in the cooked rice, beans, tomatoes, and Cajun seasoning. Pour into a casserole dish.
Cover and bake in the preheated oven for 25 minutes until hot, stirring once.