In a large skillet, heat the olive oil or butter over medium heat. Add the chopped onion and sauté for about 3-4 minutes until soft and translucent.
Add the chopped broccoli florets to the skillet and continue to cook for another 5 minutes, until they begin to soften.
In a large mixing bowl, combine the cooked rice, sautéed onions and broccoli, cheddar cheese, cream of mushroom soup, milk, garlic powder, black pepper, and salt. Stir until everything is well mixed.
Transfer the mixture into a greased 9x13 inch baking dish, spreading it evenly. If desired, sprinkle crispy breadcrumbs over the top for added crunch.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 10-15 minutes, or until the casserole is bubbly and the top is golden brown.
Remove from oven and let it cool for a few minutes before serving.
Notes
Optional crispy breadcrumbs can be added for extra texture.