Season the pork. Pat the pork dry with paper towels, then season with the salt and black pepper.
Brown the pork. Heat the oil in a large deep skillet over medium-high heat. Brown the pork in batches, leaving space between the pieces so they sear properly. Transfer to a plate; the pork will finish cooking in the gravy.
Cook the onions. Reduce the heat to medium. Add the onions to the pork drippings and cook, stirring occasionally, until softened and golden. Add 1 tablespoon butter and the garlic, then cook for about 1 minute.
Build the gravy. Stir in the tomato paste and cook for 1–2 minutes. Add the paprika and flour and stir for about 1 minute. Gradually pour in the beef stock, stirring and scraping up the browned bits from the bottom. Stir in the Worcestershire sauce.
Simmer the pork. Return the pork and any collected juices to the skillet. Add the thyme and bay leaves. Bring to a gentle simmer, cover, and reduce the heat to low. Simmer for about 50 minutes, or until the pork is tender, stirring every 10 minutes and checking that the gravy is not sticking to the bottom. Continue cooking if the pork still feels firm.
Brown the mushrooms. While the pork simmers, melt the remaining 1 tablespoon butter in a separate skillet over medium-high heat. Add the mushrooms and cook, stirring occasionally, until their released moisture evaporates and they turn golden brown.
Finish the stew. Add the browned mushrooms and Dijon mustard to the tender pork. Simmer uncovered for about 10 minutes, stirring occasionally, until the gravy thickens slightly and coats the meat. Remove the bay leaves, taste, and adjust the seasoning. Sprinkle with parsley and serve warm.