Prepare the Duxelles: In a food processor, pulse the sliced mushrooms and diced yellow onion until finely chopped. Heat a skillet over medium-high heat, add a tablespoon of butter, and sauté the mixture, stirring often.

Cook for about 5-7 minutes until the moisture has evaporated and the duxelles is slightly browned. Add in the diced sirloin, dried thyme, salt, and pepper, stirring until the beef is browned, then mix in Dijon mustard. Remove from heat and let it cool.

Roll Out Puff Pastry: Preheat your oven to 375°F (190°C). On a floured surface, roll out two sheets of thawed puff pastry until they are thin and slightly larger than your baking sheet. Press the sheets together at the seams to form one large sheet.

Assemble the Wreath: Place the prosciutto in an overlapping layer on one side of the puff pastry. Spoon the cooled sirloin mixture on top of the prosciutto and carefully roll the puff pastry into a log shape. Cut slits every 1.5 inches along the log.

Apply Egg Wash: Whisk together the egg and water to create an egg wash. Using a pastry brush, generously coat the surface of the Beef Wellington Wreath with the egg wash.

Bake the Wreath: Carefully transfer the assembled Beef Wellington Wreath onto a baking sheet lined with parchment paper. Bake for 30 minutes, or until golden brown.

Make the Red Wine Reduction Sauce: While the wreath bakes, heat a saucepan over medium heat and melt 2 tablespoons of butter. Add minced garlic and sauté for 1-2 minutes until fragrant. Pour in the beef broth and red wine, simmering for about 10 minutes until reduced by half.

Serve and Enjoy: Once the Beef Wellington Wreath is done baking, let it rest for about 10 minutes before slicing and serve with the warm red wine reduction sauce.