Cook the rice: Prepare 1 cup of long-grain white rice according to package directions (yields about 3 cups cooked). Set aside to cool slightly. If using frozen broccoli, thaw and drain well.
Prep the broccoli: If using fresh broccoli, steam or blanch florets until fork-tender (about 5–7 minutes). Drain thoroughly. For frozen broccoli, thaw and squeeze out excess water.
Make the cheese sauce: In a large saucepan, melt butter over medium heat. Whisk in flour and cook for 1–2 minutes to form a roux. Gradually whisk in milk until smooth. Cook, stirring constantly, until the sauce thickens (about 5 minutes). Remove from heat and stir in 1 1/2 cups of cheddar cheese until melted. Season with garlic powder, onion powder, salt, and pepper.
Preheat oven: Preheat your oven to 350 °F (175 °C). Grease a 9×½13-inch baking dish.
Assemble the casserole: In a large bowl, combine cooked rice, tender broccoli, and cheese sauce. Mix until evenly coated. Transfer to the prepared baking dish, spreading evenly.
Add topping: Sprinkle with remaining 1/2 cup of cheddar cheese. If using, top with crushed Ritz crackers or breadcrumbs mixed with melted butter.
Bake: Bake for 25–30 minutes, or until the casserole is bubbly and the topping is golden brown. Let rest for 5 minutes before serving.