Preheat the oven to 375°F (190°C).
Heat the olive oil in a large skillet over medium heat.
Add the diced onion and minced garlic to the skillet and sauté for 3-4 minutes until softened.
Add the ground beef to the skillet and cook until browned, breaking it up with a spoon, about 6-8 minutes.
Season the beef mixture with salt, black pepper, and dried thyme. Stir well and remove from heat.
In a mixing bowl, combine the sour cream and milk until smooth.
Lightly grease a 9×13 inch casserole dish.
Arrange half of the sliced potatoes evenly on the bottom of the casserole dish.
Spread half of the cooked ground beef mixture over the potatoes.
Pour half of the sour cream and milk mixture evenly over the beef layer.
Repeat the layers with the remaining potatoes, ground beef mixture, and sour cream mixture.
Sprinkle the shredded cheddar cheese evenly over the top layer.
Cover the casserole dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the potatoes are tender and the cheese is golden and bubbly.
Remove from the oven and let the casserole rest for 5 minutes before serving.