Step 1

Mix all the ingredients together in a small bowl. Cover and refrigerate until ready to serve. These can be made up to two days ahead.

Step 2

Preheat your oven to 400F.

Step 3

Season the beef cubes with salt and pepper. Preheat a large skillet over high heat. Add the ghee or oil, and sear the beef quickly, for up to a minute, until browned (you don't need to brown it on all sides). Remove the beef from the pan and set it in the refrigerator while you make the filling.

Step 4

Add the finely chopped mushrooms to the pan, also over high heat (this is the temperature which you should almost always cook mushrooms). Once they’ve released their liquid and are starting to brown, reduce the heat to medium and add the minced shallot. You’ll cook this for just a few more minutes until the shallot is soft, stirring frequently.

Step 5

“Drain” the filling on paper towels, remove the beef from the fridge and pat it dry.

Step 6

Line a sheet pan with parchment paper. Roll out two sheets of puff pastry to a large rectangle (add a small amount of flour if it sticks), about 10×14 inches. Cut each piece into 12 even pieces.

Step 7

Place one piece of prosciutto in the center of the puff pastry. Spoon approximately a tablespoon of the filling onto each square, then a piece of beef. Bring up two opposite corners and press them together, then bring up the remaining two corners and press all the seams together. Place them seam-side down on the sheet pan.

Step 8

Brush the wellingtons with the beaten egg yolk. If desired, sprinkle a tiny amount of coarse salt on top of each wellington. Place in the preheated oven and bake until the puff pastry is golden brown, about 20 minutes.