With a sharp knife, butterfly the pork by cutting about 2/3rds of the way down into the tenderloin (lengthwise, from the larger end to the tail end). Be careful not to cut all the way through. Open up the tenderloin, and place between 2 pieces of plastic wrap. Flatten with a meat pounder to about 1/2-inch thick. Remove the top piece of plastic, and season with salt and fresh ground black pepper to taste.
Add the rest of the ingredients to a bowl, add a pinch of salt and pepper, and mix together with a fork to combine. Spread the stuffing evenly on the pork, leaving at least 2 inches bare on the top edge. Starting at the edge closest to you, roll the pork up firmly, but without pressing out the stuffing.
Once rolled up, make a tie every inch or so with kitchen twine. Season the outside with salt and fresh ground black pepper to taste, and brown in a hot pan, in vegetable oil, for 2-3 minutes per side. Transfer to a 9 x12 baking dish and roast in a preheat 375 degree F. oven for 30 minutes. Remove and let rest for 15 minutes.
While the pork is resting, add 1 tbsp of Dijon mustard, and 1/2 cup of chicken stock to the hot baking dish, and scrap any of the drippings of the bottom with a spatula. Whisk until the mustard is incorporated, season with salt and fresh ground black pepper to taste, and pour into a small serving bowl. Untie the pork, and cut in 1/2-inch slices, and serve with some of the mustard pan sauce.