1/2 pound turnips, peeled and cut into 1-inch wedges
1 tablespoon vegetable oil
1 1/2 pounds pork tenderloin, sliced crosswise on the diagonal 1/2 inch thick
Salt and freshly ground pepper
1 tablespoon soy sauce
1 cup chicken stock or low-sodium broth
2 tablespoons plus 1 teaspoon Dijon mustard
2 tablespoons cold unsalted butter, cut into tablespoons
Fill a large, deep nonstick skillet with salted water and bring to a boil. Add the parsley and blanch until bright green, about 1 minute. Using a slotted spoon, transfer the parsley to a plate. Add the turnips and simmer over moderately high heat until tender, about 12 minutes. Drain and transfer to the plate with the parsley.
Wipe the skillet dry. Add the vegetable oil and heat until shimmering. Season the pork medallions with salt and pepper, add them to the skillet and cook over moderately high heat until richly browned, about 3 minutes per side. Transfer the medallions to a large plate.
Add the soy sauce to the skillet; when it sizzles, add the stock. Boil until reduced by half, about 6 minutes. Stir in 2 tablespoons of the mustard and reduce the heat to low. Return the pork medallions and any accumulated juices to the skillet and simmer until the pork is just pink in the center, about 1 minute. Stir in the parsley and turnips and cook until just heated through. Remove from the heat and stir in the remaining 1 teaspoon of mustard. Add the butter 1 tablespoon at a time and stir until the sauce is creamy. Season with salt and pepper. Transfer the medallions to plates, spoon the sauce, parsley and turnips on top and serve.