Rôti de Porc Poêlé: Trim all fat and sinewy silver tissue from pork tenderloin. Dry thoroughly with paper towel. Combine salt, freshly ground pepper, thyme and garlic in a small bowl; rub into the surface of the pork. Place pork in ziploc bag; press as much air out of ziploc bag as possible and seal. Refrigerate for several hours, up to 6 hours.
Preheat oven to 325° F.
Remove pork from ziploc bag and thoroughly rinse off the dry rub. Dry thoroughly with paper towels. Add olive oil to a heavy oven-proof pan with lid, just large enough to hold the pork. Place over medium high heat. When the oil is almost smoking, add the pork and brown on all sides, about 4 minutes. Remove the pork to a side dish.
Add the onions, carrots, unpeeled garlic cloves, parsley, and vermouth to the pan used to brown the pork. Reduce heat to medium low; cover pan and cook slowly for 5 minutes. Remove cover and add the pork to the pan. Insert meat thermometer into center of pork. Cover and place in the preheated oven for about 20 minutes or until the meat thermometer reaches 170° F. When done, place the pork on a hot serving platter; cover with aluminum foil. Pour the vegetables into a strainer, reserving the juices. Discard vegetables. Set reserved juices aside.
Sauce Moutarde à la Normande: Pour the vinegar and peppercorns into the same pan that was used to cook the pork, and boil until the vinegar has reduced to about a tablespoon, several minutes. Pour in the reserved meat juices and boil them down rapidly until they have reduced in half, several more minutes. Add the cream, and simmer for 5 minutes. Beat in the mustard combined with water, and simmer 2 to 3 minutes more. To thicken the sauce, add beurre meunière made with one tablespoon of softened unsalted butter and one tablespoon of flour, and continue whisking until thickened. Yield: 2 cups.
Slice the pork into 1-inch medallions and serve with the sauce. Yield: 2 to 4 servings.