Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until soft, about 3-4 minutes.
Add the minced garlic and ground beef to the skillet. Cook, breaking the meat apart, until the beef is browned and fully cooked. Drain any excess fat.
In a large mixing bowl, combine the cooked beef mixture with the cream of mushroom soup, salt, pepper, thyme (if using), and milk. Mix well.
In the casserole dish, layer half of the sliced potatoes on the bottom. Pour half of the beef mixture over the potatoes. Repeat with the remaining potatoes and beef mixture.
Cover the casserole with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and sprinkle the shredded cheddar cheese over the top.
Return the casserole to the oven and bake uncovered for an additional 15 minutes, or until the potatoes are tender and the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving.
Optional: For a variation, add frozen mixed vegetables to the ground beef mixture for extra nutrition.