Slice the pork into 1/4-inch thick medallions (about 3 ounces each). Place the pork in a large shallow dish. Add 3 cups of the oatmeal stout, the salt and pepper. Cover with plastic wrap and marinate in the refrigerator for at least 4 hours or overnight.
Prepare the potatoes: Pour the oil into a deep-fat fryer or a heavy medium saucepan. Heat the oil over medium heat to 350 degrees F. Carefully add the potatoes, several slices at a time into the heated oil and cook for 4 minutes, until crispy. Using a slotted spoon or a French fry skimmer, remove the potatoes from the oil and place on paper towels to absorb any excess oil. Reserve and keep warm in a 300 degree oven.
Remove the pork from the marinade; discard the marinade. Pat the pork dry with paper towels. In a large skillet, cook the pork in the butter over high heat for 2 minutes on each side, until brown. Remove the pork from the skillet. Reserve and keep warm.
Reduce the heat to medium and add the mustard greens, shallots, parsley and thyme. Saute for 2 minutes, until wilted. Add the remaining 1 cup oatmeal stout to deglaze the pan, loosening any browned bits on the bottom of the pan with a wooden spoon. Stir in the demi-glace and cook over medium to high heat about 5 minutes, until the liquid is reduced by half.
To serve, arrange a bed of the mustard greens mixture in the center of each plate. Top with the pork medallions and fried sweet potatoes.