Combine the milk and an equal amount of water, add salt and boil in a covered pot on low heat. When it starts to boil, add the corn flour and whisk until the polenta is done. Add the finely chopped herbs and 50g (1.75 oz) butter then put aside.
Season the pork tenderloin with salt and pepper and fry on all sides in a pre-heated pan. Transfer to a tray and bake for about 15 minutes at 180°C (355°F).
In the same hot pan used for frying the pork, add the red wine and let it reduce to a quarter of the initial quantity. Add the stock and continue to boil the reduction. Add 20g (3/4 oz) of cold butter and mix well, over high heat, until the sauce reaches the right consistency.
Slice the onion into large cubes, add the minced garlic and sauté in butter (80g or 2.75 oz), over medium heat until golden.
Add the porcini mushrooms cut into large pieces, fry briefly then add the thyme and rosemary. Continue to cook the mushrooms until they brown.