Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, brown the ground beef until fully cooked. Drain excess grease.
Add the chopped onion, bell pepper, and minced garlic to the skillet. Sauté for about 5 minutes until the vegetables are tender.
Stir in the frozen mixed vegetables, diced tomatoes (with juices), cream of mushroom soup, cooked rice, garlic powder, salt, and black pepper. Mix until well combined.
Transfer the mixture into a 9x13-inch baking dish, spreading it evenly.
Top the casserole with shredded cheddar cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes before serving.