Cut pork tenderloin crosswise into 1/2-inch (1 cm) thick slices. In large skillet, heat vegetable oil over medium-high heat; brown pork and remove to plate. To pan, add chicken stock, Dijon mustard, sage and pepper; bring to boil. Return pork to pan; simmer over medium heat until just a hint of pink remains inside, 4 minutes. Whisk cornstarch with water; add to pan and simmer, stirring until thickened.