Place cranberries in a small saucepan over medium low heat. Add water and stir together. Bring to a simmer and let simmer for 3 minutes. Drain, reserving both cranberries and cooking liquid. Set aside.
In a large skillet, heat oil over medium heat. Season pork medallions with salt and pepper and add to skillet. Saute on both sides until browned and no longer pink inside, about 3 minutes per side. Transfer to a platter, cover loosely and keep warm.
In the same skillet, add chopped shallot and cook for 30 seconds. Pour in port and vinegar and bring to a boil, stirring to scrape up any brown bits on the bottom of the skillet. Boil until liquid is reduced by half, 3 to 5 minutes. Add chicken stock, thyme and reserved cranberry liquid; boil all together until reduced by half, 5 to 7 minutes.
In a small bowl dissolve cornstarch in 1 tablespoon water and mix together. Whisk mixture into saucepan and let simmer, stirring, until sauce is slightly thickened and glossy. Stir in reserved cranberries and season with salt and pepper to taste. Spoon sauce over pork and serve.
