Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a food processor, pulse together the sliced mushrooms and diced yellow onions until finely chopped.

Heat a large skillet and add the mushroom and onion mixture. Cook for 3-4 minutes until mushrooms are moisture-released.

Add the sirloin and dried thyme, cook until beef is browned but not fully cooked. Drain excess fat and stir in Dijon mustard.

Unfold the thawed puff pastry sheets on a floured surface, roll gently to about 1/8 inch thick.

Lay prosciutto over one side of pastry, add the cooked beef and mushroom mixture, then roll up into a log shape.

Slice the rolled pastry into 1.5-inch pieces and shape into a circle on the baking sheet.

Whisk the egg and apply it generously over the pastry for a golden color.

Bake for 30 minutes or until the pastry is puffed and golden brown.

While baking, melt butter in a saucepan, add garlic, pour in red wine and beef broth, simmer and whisk in Dijon mustard.

Once cooled, transfer to a serving platter, drizzle red wine sauce around it or serve on the side.