• In a food processor, pulse mushrooms and onions into fine pieces (duxelles), or finely dice by hand.
• In a skillet over medium-high heat, cook sirloin, mushrooms, onions, and thyme. Season with salt and pepper. Brown for 3–4 minutes, drain grease, then stir in Dijon mustard.
• Preheat the oven to 375℉. Line a baking sheet with parchment paper or a silicone baking mat.
• Roll out both puff pastry sheets and press the seams of the two sheets together to form one long sheet. You can place the parchment paper underneath the puff pastry now or you can wait until it's in a circle, whichever is easier for transferring for you.
• On one side of the sheet, layer prosciutto, then the sirloin mixture. Roll into a log. Slice every 1.5" about ⅔ through, then curve into a circle and seal ends. Twist each cut piece so that the inside filling is facing up. Place it on parchment paper and place it onto a baking sheet if you have not already done so.
• Brush with egg wash (1 large egg + 1 tablespoon water). Bake for 30 minutes.

Red Wine Reduction

• Add a tablespoon of butter to a saucepan. Add garlic and thyme and cook for 1 minute.
• Pour in red wine and beef broth. Simmer until reduced by half and thickened.
• Stir in Dijon mustard. Remove from heat and add another tablespoon of butter. Whisk until smooth.
• Add black pepper to taste. Serve warm as a dipping sauce for beef Wellington wreath.