Potatoes
In a large frying pan, heat the vegetable oil until it reaches 250�F. Cook the potato slices, in batches, until cooked through. Remove, and drain on a paper towel.
Meat Filling
In a saucepan over medium heat, add the butter and olive oil. Next, add the onion, carrot, and bay leaf, cook until they have softened. Add the ground beef, and saut� until browned. Season with salt and pepper then pour in the beef stock, Worcestershire sauce, and tomato pur�e. Bring to the boil, then reduce the heat and simmer for 30 minutes, stirring occasionally.
Once cooked, the meat mixture shouldn�t be wet at all. If it is turn up the heat, and allow to come to a boil for a few minutes. Remove from heat, and set aside to cool.
Assemble
Place a piece of Saran wrap over a small bowl and place potato slices in a circle, overlapping them in a fan shape. Make sure there are no holes.
Add a small spoonful of the meat filling in the center of the potato circle, and then begin to bring the potato slices up around the filling. Pull the Saran wrap up and around the potato to seal, and place the stuffed potato bomb on a plate. Repeat this step until all the filling is used. Place the potato bombs in the fridge for at least 30 minutes.
Preheat the oven to 350�F.
When ready to cook, unwrap the bombs from the Saran wrap. In a baking dish, place the potato bombs, and ensure they fit tightly together. Pour the b�chamel sauce on top, and sprinkle the cheese. Place in the oven, and bake for 20 minutes until the cheese is golden and bubbling.