Season the beef filet with salt and pepper and sear on all sides until nicely browned.
While the beef is still warm, brush it with Dijon mustard and set aside.
Sauté the mushrooms, shallots, garlic, and thyme until all the moisture cooks out.
Deglaze the pan with white wine, then stir in the beef broth concentrate.
Add truffle paste or truffle slices, if using.
Lay out the prosciutto slices and spread the mushroom mixture evenly over the top.
Place the beef filet on top of the mushroom mixture and roll everything up snugly.
Wrap the prosciutto-covered filet completely in the thawed puff pastry.
Refrigerate at this point if preparing the Beef Wellington ahead of time.
Bake at 375°F for 25–35 minutes, until the pastry is golden and flaky.
Allow the Beef Wellington to rest briefly before slicing and serving.