In a large skillet, heat the olive oil or butter over medium heat. Add the finely chopped onion and sauté for about 3-4 minutes until soft and translucent.
Add the chopped broccoli florets to the skillet and sauté for an additional 5 minutes until vibrant and slightly tender.
In a large mixing bowl, combine the cooked rice, sautéed onions and broccoli, shredded cheddar cheese, cream of mushroom soup, and milk. Add garlic powder, black pepper, and salt. Mix thoroughly.
Transfer the mixture into a greased 9x13 inch baking dish, spreading it out evenly. If desired, sprinkle breadcrumbs on top.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes until bubbly and golden brown.
Allow the casserole to cool for a few minutes before serving.
Notes
Serve hot, garnished with fresh parsley or green onions for added color.