Preheat the oven to 425 degrees Fahrenheit. Line a baking sheet with parchment.
3 pounds Beef Tenderloin (fat trimmed)
Salt and Pepper to taste
Remove steak from the fridge and let come to room temperature, about 30 minutes before you're ready to cook it. Season steak with a generous amount of salt and pepper. Heat a large saute pan over medium-high.
3 tablespoons Extra Virgin Olive Oil
Add about 3 tablespoons of oil to the pan and sear meat on all sides until golden brown, about 10 minutes. Drain the excess fat from the pan.
4 tablespoons Butter (divided)
2 Shallots (minced)
2 Garlic cloves (minced)
Heat another large saute pan over medium-high. Add 3 tablespoons of butter, and once melted and foamy, add the shallots and a pinch of salt and cook for 2 to 3 minutes, until softened. Add garlic and cook just until fragrant, about 1 minute.
1 1/2 pounds Button Mushrooms (minced)
3 fresh Thyme sprigs (leaves pulled)
Add the mushrooms and thyme, season with salt and pepper and toss to incorporate. Saute for 5 minutes, stirring occasionally, until the mushrooms are cooked and there is little moisture remaining in the pan. Remove from heat and allow to cool.
2 pieces Puff Pastry (thawed)
1/3 cup English Mustard
Roll out the pastry on a lightly floured surface to about 3 to 4 inches longer and 5 to 6 inches wider than the tenderloin. Brush with the mustard and spread the cooled mushroom mixture onto the pastry, leaving a one inch border.
1 Egg plus 1 tablespoon of Water (whisked)
Place the tenderloin into the center of the pastry. Fold the dough around the beef to cover completely and seal by pressing gently. Transfer to the prepared baking sheet, seam-side down. Brush with the egg wash and season with salt.
Bake for 30 to 35 minutes, until the pastry is a nice golden brown and the internal temperature reaches 125 to 130 degrees Fahrenheit for medium rare. Remove from pan and allow to rest.
1/4 cup Red Wine
1/2 cup Beef Stock
1 tablespoon Creme Fraiche
Return the original saute pan over a medium heat. Once hot, deglaze with red wine, scraping up the crispy bits with a wooden spoon. Whisk beef stock, remaining butter and the creme fraiche just until warm and emulsified. Adjust the seasoning to taste and serve over sliced tenderloin.