Step 1

In a food processor, add chopped mushrooms and onions and pulsate until blended into fine pieces.

Step 2

Heat 1 tablespoon of olive oil in a large pan over medium-high heat. Once hot, add chopped sirloin, mushrooms, onions, and thyme. Season with salt and pepper to taste. Cook 3-4 minutes until beef is browned, drain if necessary, stir in dijon mustard, and set aside.

Step 3

Preheat oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper.

Step 4

On a clean surface, roll out both puff pastry sheets side by side. Cut each sheet in half from left to right and top to bottom to create 4 equal squares on each sheet.

Step 5

Place 1 slice of prosciutto diagonally on each square (will likely need to be folded in half to fit.) Spoon beef mixture into the center of each square, diagonally so that it goes from the top corner to the bottom corner. See photos above. Close puff pastry by lightly wetting two opposite corner tips and overlapping, pressing to adhere so they do not open during baking.

Step 6

Place turnovers on the prepared baking sheet. Scramble egg and water in a small dish and brush each puff pastry with egg wash. Bake for 20-25 minutes until golden brown.

Step 7

While they bake, prepare the sauce by using a saucepan and whisking all ingredients except the cold water and corn starch. Bring to a simmer for about 8 minutes, stirring often. Mix cornstarch and cold water in a small bowl, and add to the saucepan over high heat while constantly stirring as it thickens. Serve with turnovers as a dipping sauce or drizzle over top.