Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
In a large skillet, heat the oil over medium heat. Add the ground beef, onions, and garlic. Cook until the beef is fully browned, breaking it up with a spatula.
Drain off any excess grease. Season the mixture with salt and pepper. Remove from heat.
2. Make the Sauce
In a medium bowl, whisk together the condensed cream of soup and milk until the mixture is smooth.
3. Layer the Casserole
First Potato Layer: Arrange about one-third of the thinly sliced potatoes in an even layer on the bottom of the prepared baking dish.
Beef Layer: Spread half of the seasoned ground beef mixture over the potatoes.
Cheese Layer: Sprinkle 1/2 cup of the shredded cheese over the beef.
Repeat: Place the next one-third of the potatoes, followed by the remaining ground beef.
Top Potato Layer: Place the final layer of potatoes on top.
Pour Sauce: Pour the cream of soup mixture evenly over the entire casserole, ensuring the sauce covers the potatoes, allowing it to seep down.
4. Bake
Cover the baking dish tightly with aluminum foil.
Bake in the preheated oven for 60 minutes. (The potatoes should be soft when pierced with a fork).
Carefully remove the foil.
Sprinkle the remaining 1.5 cups of cheese over the top.
Return the casserole to the oven, uncovered, and bake for another 10-15 minutes, or until the cheese is melted, bubbly, and golden brown.
5. Serve
Let the casserole rest for 10 minutes before slicing and serving.