Place the flour, garlic salt and pepper into a plastic bag. Shake to mix, add the pork tenderloin pieces and shake again to coat. Shake off the excess flour.
Heat the oil in a large frypan over medium-high heat. Cook the pork medallions in the hot oil until golden-brown on both sides, 2 to 3 minutes per side. Pour in the balsamic vinegar and chicken stock. Bring to a boil then reduce heat to medium and simmer until the pork is no longer pink in the centre, 3 to 4 minutes. Remove the pork to a serving platter then stir the lemon zest and capers into the simmering sauce. Continue simmering until the sauce has thickened to your desired consistency.