For the pastry, add the flour and salt to a bowl and mix. Use a box grater to grate in the butter, dipping the butter into the flour before each stroke so as to keep the flakes separated. Using your hands, mix together briefly, then add a little cold water. Continue to mix gently as you add cold water, until the mixture forms a dough. Do not overmix: keep the butter well distributed but not fully incorporated, which will make the pastry flaky. Wrap pastry in plastic wrap and let rest in refrigerator for 1 hour.
For the tenderloin, remove from fridge and season generously with salt. Add a little olive oil to a frying pan large enough to fit the tenderloin, then heat over medium-high. When the oil starts to smoke lightly, add tenderloin to the pan and sear on all sides.
Remove tenderloin from the pan and let cool in fridge.
To make the duxelle, add the tallow to a pan and heat over medium until melted. Add shallots and garlic and cook until sweating. Add the sage, thyme, pepper, and fresh mushrooms and stir. Add the dried creminis and white wine and continue to cook until liquid has evaporated and the mixture is dry. Add cream and cook until mixture is once again dry.
Transfer the mixture to a chopping board, then use a sharp chef's knife to chop through the duxelle until it makes a coarse paste. Transfer to a bowl and season with salt. Let cool.
Remove the tenderloin from the fridge, spread mustard over top, then roll in the duxelle, which will stick to the beef. Return to the fridge.
Preheat oven to 400 F (205 c).
Take the pastry out of the fridge and roll out to the thickness of a loonie. Cut out a sheet of pastry that is about 4-inches (10 cm) longer than the length of the tenderloin. The width of the pastry should be large enough to wrap around the entire circumference of the beef. Once the pastry is cut out, remove the tenderloin from the fridge and place in the very middle of the pastry. Cut off the 4 corners of the pastry—the cuts should be square-shaped and should reach only to the edges of the beef. You should have what looks like 2 tabs remaining on both short sides of the beef.
Brush all edges of the pastry with the eggwash.
Lift the tabs on the short sides of the pastry and fold up over the beef. Then pull the long flap closest to you over the top of the beef and roll the whole tenderloin over until the two long edges of the pastry connect with each other.
With the remaining rolled out pastry, cut another piece that is the same length as the original piece that was cut to wrap the beef. Roll the lattice pastry cutter over the piece of pastry to create a lattice.
Brush eggwash over the whole of the wrapped wellington pastry, then lift the lattice and place over top of the beef wellington. Spread the pattern/lattice out evenly over the wellington, then tuck in the excess on both sides under the bottom of the wellington. Brush the pastry all over with egg wash again.
Take the micro sprigs of thyme and place in the holes of the lattice.
Place in the oven and bake for roughly 40 minutes, or until the internal temperature of the beef is 125 F (52 c).
Remove from oven, then let the wellington rest for 20 minutes before slicing and serving.
This recipe is used with permission and is owned by Marben restaurant.