Preheat the oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Add the ground beef to the skillet. Cook until browned, breaking it apart with a spoon. Drain any excess fat.
Stir in the garlic powder, salt, and black pepper. Mix well.
In the baking dish, layer half of the sliced potatoes on the bottom.
Spread the cooked beef mixture over the potatoes.
Top with frozen mixed vegetables.
Pour the cream of mushroom soup evenly over the vegetables.
Layer the remaining potato slices on top, covering the casserole.
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and sprinkle the shredded cheddar cheese on top.
Bake for an additional 15 minutes or until the potatoes are tender and the cheese is melted.