Cut the pork tenderloin crosswise into rounds about 1/2 inch thick. Blot the medallions thoroughly with paper towels, then season both surfaces with salt and black pepper.
Place the flour in a shallow dish and season it with a small amount of salt and pepper. Coat each pork medallion lightly on both sides, then shake away the excess flour. This thin coating encourages an evenly browned exterior during searing.
Finely mince the garlic and parsley. Keep them separate and set both aside until needed.
Place a large skillet over medium-high heat. Add the olive oil along with 1 tablespoon of the butter, heating until the butter begins to foam and the skillet is thoroughly hot.
Arrange the flour-coated pork medallions in the skillet without crowding them, cooking in batches when necessary. Sear for approximately 3 minutes on each side, leaving the pieces relatively undisturbed so that a deep golden crust can develop.
Transfer the browned pork to a clean plate. The medallions do not need to be completely cooked at this stage because they will finish cooking in the sauce.
Keep the same skillet on the heat without cleaning it, preserving the browned bits left behind from the pork. Add the minced garlic and cook for approximately 30 seconds, stirring continuously, just until fragrant. Do not allow the garlic to brown.
Stir in the Dijon mustard, chicken broth or dry white wine, and herbes de Provence. Allow the mixture to simmer for about 1 minute, giving the liquid time to reduce slightly while the flavors combine.
Reduce the heat to medium. Gradually pour the heavy cream into the skillet while stirring continuously to produce a smooth sauce.
Maintain a gentle simmer for approximately 2 minutes, stirring from time to time, until the sauce thickens slightly and is able to coat the back of a spoon.
Return the seared pork medallions to the skillet and arrange them in a single layer. Turn each piece gently so that it becomes coated with the creamy herb sauce.
Simmer the pork in the sauce for approximately 5 minutes, turning the medallions once midway through cooking. Continue until the pork is cooked through, tender, and firm without becoming tough.
Arrange the finished pork medallions on individual plates or a serving platter, either in one layer or with the pieces slightly overlapping.
Spoon the warm creamy herb sauce generously over and around the pork so that every medallion is well coated.
Scatter the freshly minced parsley over the finished dish immediately before serving, adding fresh herbal flavor and color to the rich cream sauce.