In the meantime process the mirepoix (onion, carrots and celery) in a kitchen robot so to have them finely minced. Add the corn flour and pulse a few extra times to combine.
Warm up the oven to 180C/360F.
In the bowl of your standing mixer (equipped with the paddle attachment) combine the ground meat with the black pepper, salt, egg, corn flour and minced mint. Add the minced mirepoix, sautéed bacon and mushrooms and combine thoroughly.
To test for seasoning, take a little dollop of the meatloaf mixture (1tsp or so) and cook it as a miniature hamburger in a warm skillet.
For the filling, in a bowl combine the diced feta cheese with the pulverized herbes de Provence and the corn flour.
Oil slightly a baking tray and spread half of the meatloaf mixture so to form a rectangle of roughly 1cm/0.5" in thickness. Place the feta filling in the center, leaving a margin of at least 1cm/0.5" all around. Cover with the rest of the meatloaf mixture and seal properly with the base sheet.
Give the meatloaf a rounded top or a flat one to your liking and sprinkle it generously with grated Parmesan. Bake it for 30-40min or until a thermometer inserted in the meatiest part registers 70C/160F. If the top hasn't developed a crust yet, turn on the grill to its maximum setting, move the meatloaf pan closer to it and let glaze for few minutes.
Let the meatloaf cool down slightly before serving.
This meatloaf is best served with a basil tomato sauce livened up with some lime juice.