Cut the pork, widthwise, into 4 equal pieces. Place a piece of the pork on a cutting board and cover with plastic wrap. Gently pound or firmly press the pork into a medallion about 3/4" thick. Repeat with the other pieces of pork.
Heat the oil in a skillet over medium-high. Season the pork with salt and pepper and cook 2 to 3 minutes on each side, or until just cooked through. Transfer pork to a plate. Add the apples and sage to the skillet and sauté 1 minute. Add the brandy and simmer 30 seconds. Add the stock/flour mixture, bring to a simmer, and simmer until a lightly thickened sauce forms. Season the sauce with salt and pepper. Return pork to the skillet, heat through a minute or so, and then serve.