For mushroom mixture:
1 pound mushrooms, chopped
1 small onion chopped
1 tablespoon extra-virgin olive oil
1/2 teaspoon salt
1/4 teaspoon black pepper
1/2 cup all-purpose flour
1 teaspoon sugar
1/2 teaspoon salt
2 tablespoons chives, minced
1/2 cup whole milk
1/4 cup water
4 tablespoons butter, melted
For the Wellington:
2 pounds filet mignon
Salt and black pepper, to taste
2 tablespoons olive oil
1/4 cup Dijon mustard
1/2 pound prosciutto
1 sheet puff pastry, thawed
1 egg yolk
1 To make mushroom mixture: Add mushrooms and onions to a food processor and process until completely smooth.
2 In a pan over medium heat, add mushroom/onion paste, olive oil, salt, and pepper. Spread the mixture evenly over the surface and cook on a medium-low heat until the moisture in the paste has reduced and the mixture has the consistency of a spreadable pate. Remove from heat and let cool.
3 To make the crepes: In a large bowl, whisk together flour, eggs, sugar, salt, and chives. Whisk in milk, water, and 1 tablespoon of melted butter, until smooth and emulsified.
4 Heat a crepe pan or large skillet over medium heat. Add remaining 3 tablespoons of butter. Add 1/4 cup of the batter. Rotate pan in a circular motion over the heat to completely cover the surface with the batter. The edges of the crepe will begin to curl slightly as the crepe cooks. Cook for approximately 45 seconds, then flip crepe to cook the other side. Each side should be a pale golden brown. Set crepes aside.
5 Preheat the oven to 400º F.
6 To make filets: Pat dry and season generously with salt and black pepper. Heat a skillet over medium-high heat and add evoo. When the oil begins to smoke, add the filet and brown from 1 to 2 minutes. You want to create a nice sear on the outside of the filet but leave the inside raw. Remove from heat and place on a cutting board to cool.
7 Cover cooled filet with Dijon mustard.
8 To roll the beef Wellingtons: On a cutting board, lay out a long piece of plastic wrap. In the middle of the wrap, lay out a crepe. Spread mushroom mixture over the crepe. Lay out the prosciutto on top of the mushroons. Place filet in the center of the crepe and gently wrap the crepe around the filet. Trim off any excess and use the plastic wrap to tightly wrap the steak.
9 Lay out a clean, long piece of plastic wrap. Gently roll out puff pastry until it is a 1/4-inch thin. Place the wrapped filet on one end of the puff pastry and wrap. Pinch the ends closed and trim off any excess puff pastry. Use the plastic wrap to tightly seal the puff pastry. Pop it in the fridge for about 5 minutes to let it firm up.
10 In a bowl, mix together egg yolk and milk.
11 Place a sheet of parchment paper or aluminum foil on a baking sheet. Remove Wellington from fridge, remove the plastic wrap, and lay the Wellington seam down on the baking sheet. Baste the top of the puff pastry with the egg wash and bake for approximately 25 to 30 minutes or until the internal temperature of the filet reaches 125º F, or to whatever temperature you prefer your filet.
12 Remove from the oven and let rest for at least 10 minutes before slicing into medallions. Serve warm.