• If roast is tied, remove twine. Rub and season all over with olive oil, salt and pepper. Heat 1 tbsp of the butter with the vegetable oil in heavy skillet over medium-high heat; brown meat on all sides, about 8 minutes. Set aside.
  
• In same skillet, melt remaining butter; add shallots and onion and cook over medium-high heat, stirring, until softened for about 1 minute. Add mushrooms and cook, stirring occasionally, until there is no liquid from the mushrooms remaining, about 5 to 10 minutes. Season to taste and set pan aside to use for the Madeira Mushroom Sauce
  
• Pat reserved tenderloin dry with paper towel; top with pâté slices and mushroom mixture, pressing mixture onto tenderloin to hold. Set aside. Line rimmed baking sheet with foil or parchment paper; spray lightly with non-stick spray. Set aside.
  
• Roll out the package puff pastry dough on lightly floured board, on its underside, into a 13 x 12-inch (32 x 30 cm) rectangle, rolling over the seam to hold the 2 squares together. Place oven tenderloin and tuck dough around the sides and ends.
  
• Gently turn over, wrapping dough around underside and ends of tenderloin to fully enclose; brush some beaten egg along edges of seams where dough overlaps, pressing gently to seal.
  
• Place seam side down onto baking sheet. Refrigerate for at least 15 minutes or up to 1 hour. (If refrigerated 1 hour, let the Wellington sit at room temperature 30 minutes before baking.)
  
• Adjust oven rack to lowest position and heat oven to 450°F (230°C) oven. (NOTE: The Beef Wellington is roasted on the lowest rack of a very hot oven to help create a crispy crust.) Cut 3 slits across the top of pastry to allow steam to escape while baking; brush all over with beaten egg.
  
• Bake Wellington until golden brown (shielding with loose cover of aluminum foil if getting to brown), and digital instant read thermometer inserted into centre of roast reads 145°F (63°C) for medium-rare doneness, about 35 minutes. Let stand for 10 minutes and transfer to carving platter. Carve into thick slices and serve with Madeira Mushroom Sauce.

Madeira Sauce

• Place skillet used to make mushrooms over medium-high heat; stir in beef broth, stirring up brown bits from bottom of pan. Bring to boil and cook for 15 to 20 minutes to reduce to 1-2/3 cups. Stir in Madeira and continue to boil until liquid is reduced again to 1-2/3 cups about 5 minutes. Stir in whipping cream to finish. If desired, thicken by stirring in corn starch mixture; cook 3 to 4 minutes until slightly thickened. Season with salt and pepper to taste. Pour into gravy boat to serve.