Preheat your oven to 350°F (175°C).
In a large skillet, brown the ground beef over medium heat until fully cooked.
Drain excess fat from the skillet.
Add chopped bell peppers and diced onions to the skillet and sauté until softened, about 5 minutes.
Stir in the frozen mixed vegetables, cream of mushroom soup, garlic powder, salt, and pepper.
Mix in the cooked rice until everything is well combined.
Transfer the mixture to a greased 9x13 inch baking dish.
Top with shredded cheddar cheese evenly across the casserole.
Bake in the preheated oven for 30 minutes or until the cheese is bubbly and golden.
Let the casserole cool for a few minutes before serving.