Start by making the pork croquettes. Heat a medium non-stick frying pan over medium heat. Cook bacon for 3 to 4 minutes or until crisp. Drain on a plate lined with paper towel. Combine mashed potato, egg yolk, onion, bacon and flour in a large bowl. Form the mix into a semi-circle shape then add the diced cheese in it and close it with the remaining mix to form a round shape. Place remaining flour on a plate. Place breadcrumbs on another plate. Whisk egg and milk together in a shallow bowl. Roll croquettes in flour, shaking off excess. Dip in egg mixture. Coat in breadcrumbs. Heat oil in a deep frying pan over medium-high heat. Cook croquettes, in batches, turning, for 3 to 4 minutes or until golden. Using a slotted spoon, transfer to a plate lined with paper towel. To prepare the pork fillet, place the oil into a frying pan on a medium heat. After the oil melts, place the pork tenderloin pieces into the pan. Leave the pork to cook on its side. Once that side is cooked, turn using tongs to cook the other sides. Keep turning and cooking until the pork looks cooked on all sides. Cook all sides of the pork until the meat thermometer shows an internal temperature of just below (63C). The pork will keep on cooking a bit after you take it out of the pan. Finally prepare the red wine jus. Sauté the shallots in a medium saucepan with the oil over a high heat for about 3 minutes until lightly browned, stirring often. Season with ground black pepper and add the garlic and rosemary. Continue cooking for a further 3 minutes, stirring often to prevent the shallots burning. Pour in the vinegar and cook until evaporated away to a syrup, then pour in the wine and cook until reduced by two thirds. Pour in the stock and bring to the boil. Turn down the heat and simmer until reduced by two-thirds again, to around 250ml. Remove the garlic and rosemary. Add a little salt to taste and finally whisk in a knob of butter. Add any juices from the steaks just before serving.