With a sharp knife trim any silverskin from the pork tenderloin and cut into 1/2-inch thick slices. Pound the slices to roughly a 1/4-inch thick. Pat the pork pieces dry then season with salt and pepper on both sides.
Heat a heavy pan to medium-high heat and add the olive oil and 2 tablespoons of butter. Once the butter is foaming, sear the pork until browned (about 2 minutes per side) working in batches to not overcrowd the pan. Place the seared pork on a plate and tent with foil.
Add the mushrooms and remaining butter to the pan and saute until they release their water and brown. Season the mushrooms with a touch of salt and pepper.
Turn the heat down to medium and add the shallot to the pan and cook until soft (about 3 minutes) then add the garlic and cook until fragrant. Add the flour to the pan and cook for 1-2 minutes or until all of the white specks are gone.
Add the brandy and turn the heat to high. With a wooden spoon scrape the bottom of the pan to dislodge all of the brown bits. Boil for 30-60 seconds or until most of the liquid evaporates.
Whisk in the chicken stock and cream and bring to a boil. Once boiling, lower the heat to a simmer and add the Dijon, Worcestershire sauce, and allspice and mix to incorporate. Simmer for 3-5 minutes or until the sauce coats the back of a wooden spoon. If the sauce is too thick, just add a few ounces of stock to thin it out.
Add the pork to the sauce and cook just until the pieces are hot (about 1-2 minutes).
Remove the pan from the heat and taste the sauce. Adjust salt and pepper to taste. Mix in the parsley and serve. Enjoy!