Preheat your oven to 375°F (190°C). This will ensure your casserole bakes evenly and gets that golden, bubbly top.

Cook the ground beef: In a large skillet, heat a bit of oil over medium heat. Add the ground beef and cook, breaking it apart with a spatula, until it's browned and fully cooked through. Drain any excess fat and return the beef to the pan.

Add the onions: Toss in the chopped onion with the beef and cook for about 2-3 minutes, or until the onions become soft and translucent. This step adds a savory base to the dish.

Stir in the seasonings: Add the garlic powder, dried oregano, salt, and pepper to the beef mixture. Stir well to coat the beef and onions in the spices, letting everything come together for an extra boost of flavor.

Add the tomato sauce: Pour in the tomato sauce and stir it into the beef mixture. Let it simmer for about 5 minutes, so the flavors can meld together and the sauce thickens a bit.

Mix in the vegetables: Add the frozen mixed vegetables to the skillet and stir to combine. They’ll cook through as the casserole bakes, adding color and nutrition to the dish.

Assemble the casserole: Transfer the beef and vegetable mixture into a greased 9x13-inch baking dish, spreading it out evenly. Sprinkle the shredded cheddar cheese evenly over the top.

Top with breadcrumbs: Sprinkle the breadcrumbs over the cheese. This will create a crunchy, golden topping as it bakes in the oven.

Bake the casserole: Place the casserole in the preheated oven and bake for about 20-25 minutes or until the top is golden brown and the cheese is bubbly.

Serve and enjoy: Once the casserole is out of the oven, let it cool for a few minutes before serving. Scoop out generous portions and enjoy this comforting, easy-to-make meal.