Preheat oven to 375°F. Spray a 9x13 inch baking dish with cooking spray, making sure to coat all sides well.

Brown ground beef with onion and garlic in a large skillet over medium heat, breaking up the meat as it cooks. Cook until meat is no longer pink, about 5-7 minutes. Season with salt and pepper to taste. Drain off excess fat.

Layer half of the sliced potatoes in the bottom of the prepared baking dish, slightly overlapping each slice. Season this layer with half of the salt, pepper, and garlic powder.

Spread the cooked ground beef mixture evenly over the potato layer, making sure to reach all edges of the dish.

Arrange the remaining potato slices on top of the beef in an overlapping pattern. Season with the remaining salt, pepper, and garlic powder.

In a medium bowl, whisk together both cans of soup and the evaporated milk until smooth. Pour this mixture evenly over the entire casserole, spreading with a spatula if needed.

Sprinkle the shredded cheddar cheese evenly over the top, covering the entire surface.

Cover the dish tightly with aluminum foil, crimping the edges to seal. Bake for 1 to 1.5 hours, or until potatoes are fork-tender when tested.

Remove foil and sprinkle French fried onions over the cheese. Return to oven uncovered and bake for 5-10 minutes until onions are golden brown.

Let the casserole rest for 5-10 minutes before serving to allow the sauce to set. Cut into squares and serve hot.