Preheat the oven to 350°F.

Slice the potatoes into thin slices, about ⅛ inch thick. A mandolin slice works great for this. Place the potato slices in a bowl of cold water to prevent them from turning brown while you cook the ground beef. 

Heat a large 12-inch skillet over medium high heat. Add the ground beef and cook until browned, breaking it up as it cooks. Use a spoon or turkey baster to remove all but 1 tablespoon of the grease.

Add the onion and garlic and cook until the onion is soft.

Add the butter and stir until melted then stir in the flour. Cook, stirring for 1 to 2 minutes. Stir in the Dijon, Worcestershire, and tomato paste. Use the back of a spoon to mash the tomato paste and blend it into the mixture. 

Slowly pour in the stock or broth while constantly stirring. Pour in the milk and bring to a gentle simmer. Remove from heat and stir in the sour cream and half of the shredded cheese. 

Drain the water from the potato slices and pat them dry. Spread half of the potatoes over the top of the ground beef mixture. Sprinkle the potatoes with a bit of salt then stir them into the beef mixture. Repeat withe the rest of the potatoes. 

Sprinkle the rest of the shredded cheese over the top. Cover the skillet and slide it into the oven. Bake for 75 minutes or until the potatoes are tender, then remove the lid. Bake uncovered for 10 to 15 minutes or until the top is golden brown.