Season the beef tenderloin generously with salt and pepper on all sides. Heat the olive oil in a large skillet over high heat. Once the pan is smoking hot, sear the beef quickly on all sides, including the ends, until a brown crust forms. Remove the beef from the pan and immediately brush it all over with the English mustard while it is still warm. Set it aside to cool completely.
In the same skillet or a clean one, melt the butter over medium heat. Add the finely chopped mushrooms, garlic, and thyme. Cook the mixture for about 10 to 15 minutes, stirring frequently, until all the moisture has evaporated and the mushrooms form a thick paste. This step is crucial to prevent the pastry from getting soggy. Season with a pinch of salt and let the mixture cool completely.
Lay a large piece of plastic wrap on your work surface. Arrange the turkey bacon slices on the plastic wrap in a slightly overlapping rectangle that is large enough to wrap around the beef. Spread the cooled mushroom mixture evenly over the turkey bacon, leaving a small border at the edges.
Place the seared beef tenderloin at one edge of the mushroom and bacon layer. Use the plastic wrap to tightly roll the bacon and mushrooms around the beef, creating a neat log shape. Twist the ends of the plastic wrap to tighten it further. Place the log in the refrigerator for at least 20 minutes to firm up, which helps it hold its shape during the final assembly.
On a lightly floured surface, roll out the puff pastry sheet into a rectangle large enough to enclose the beef. Unwrap the chilled beef log and place it in the center of the pastry. Brush the edges of the pastry with the beaten egg. Fold the pastry over the beef, trimming any excess, and press the seams to seal. Place the Wellington seam-side down on a baking sheet lined with parchment paper.
Brush the entire outside of the pastry with the remaining egg wash. You can lightly score the top with a knife for a decorative Christmas pattern. Bake in a preheated oven at 400 degrees Fahrenheit for 25 to 30 minutes, or until the pastry is deep golden brown and the internal temperature of the beef reaches your desired level of doneness. Let the Easy Beef Wellington rest for 10 minutes before slicing into thick rounds.
