Step 1: Preheat oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: In a large skillet over medium-high heat, brown the ground beef, breaking it up with a spoon. Drain any excess fat.
Step 3: Add the chopped onion and minced garlic to the skillet with the ground beef. Sauté for 3-5 minutes until the onion is softened and translucent. Season with 0.5 tsp salt, 0.25 tsp black pepper, and garlic powder. Stir well.
Step 4: In a large mixing bowl, combine the browned ground beef and onion mixture with the drained corn, drained green beans, and cream of mushroom soup. Stir until well combined.
Step 5: Arrange half of the thinly sliced potatoes in an even layer at the bottom of the prepared baking dish. Season with the remaining 0.5 tsp salt and 0.25 tsp pepper.
Step 6: Spoon the entire ground beef and vegetable mixture evenly over the potato layer in the baking dish.
Step 7: Arrange the remaining thinly sliced potatoes over the ground beef mixture, creating a final top layer.
Step 8: Cover the baking dish tightly with aluminum foil. Bake for 45 minutes to 1 hour, or until the potatoes are fork-tender.
Step 9: Remove the foil. If using, sprinkle the shredded cheddar cheese evenly over the top. Return to the oven, uncovered, and bake for another 10-15 minutes, or until the cheese is melted and bubbly and the top is lightly golden.
Step 10: Let the casserole rest for 10-15 minutes before serving to allow it to set. Serve hot.