Gather the ingredients.

Cook lean ground beef and onion over medium heat until beef is browned and onion is soft and translucent, 6 to 7 minutes. Add garlic and cook until fragrant, about 1 minute. Season with salt and pepper.

Stir soup and milk together in a bowl.

Line a slow cooker with a slow cooker bag. Place a layer of potatoes, overlapping as needed, at the bottom of slow cooker. Season with salt and pepper. Top with a layer of ground beef, a layer of soup mixture and 1 cup cheese. Repeat layers with remaining ingredients.

Cook on High until potatoes are fork tender, 3 to 4 hours.