Make a row of deep cuts in each potato from end to end, they should be just under ¼ inch apart. However, take care not to cut the potatoes all the way through. Tip: You can use the trick in the video and place the potatoes between two wooden spoons, and only make the first cut after that. The spoons will act as a block for the knife. Next, place the potatoes in a cast-iron pan or a casserole dish, brush them with oil and sprinkle some salt on top. Place the pan in a convection oven for 60 minutes at 360°F. Let the potatoes cool a little and then stick the slices of cheese in the gaps.
Fry the beef in some oil and add the chopped onions and garlic. Once you've mixed it all together, stir in the tomato paste and the stock, and let the mixture cook down, until it's only slightly liquid. Season the mixture with salt and pepper to finish.
Divide the filling among the potatoes, which should still be in the pan or casserole dish. Then pour the béchamel sauce evenly on top. For the finishing touch, place a slice of mozzarella on top of each potato.
Slide the potatoes back in the oven on convection mode and bake them for another 30 minutes at 360°F.