In a large skillet over medium heat, cook ground beef until browned. Add chili powder, oregano, garlic powder, salt, and pepper. Stir to combine and set aside.
In a greased 9x13-inch baking dish, layer the cooked rice. Top with beef mixture, mushrooms, and sliced green pepper.
In a saucepan, whisk together sugar, cornstarch, water, vinegar, ketchup, salt, and pepper. Cook over medium heat until bubbling and thickened, stirring constantly. Continue to cook for 2 more minutes.
Pour the sweet and sour sauce evenly over the casserole in the baking dish.
Bake uncovered for 30 minutes, or until bubbly and heated through. Serve hot.
Notes
You can swap out green pepper for red or yellow for a sweeter bite. Add pineapple chunks for a tropical twist!