Preheat the oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes.
Add the ground beef to the skillet and cook until browned, breaking it apart with a spoon, about 5-7 minutes. Drain any excess fat.
Stir in the diced tomatoes (with juice), cream of mushroom soup, garlic powder, onion powder, salt, black pepper, and milk. Mix well.
If using, add the frozen mixed vegetables and cooked rice to the skillet. Stir until everything is evenly combined.
Transfer the mixture to a greased 9x13-inch baking dish. Spread it out evenly.
Sprinkle the shredded cheddar cheese on top.
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let it cool for a few minutes before serving.