Gather your Duxelles ingredients.
Use a food processor to chop up the mushrooms and onion.
I went very fine here, but you could do it by hand it you wanted a bit more coarse texture.
Melt the butter and oil in a skillet. Add the mushroom paste.
Add the garlic, thyme, and brandy. Cook until the brandy is mostly gone.
Add the cream and soy sauce.
Cook until the Duxelles is a nice, spreadable consistency. Place into refrigerator to cool.
Gather your beef ingredients.
Heat some olive oil in a cast iron pan on high heat. Sear your tenderloin.
Rotate the beef so that it is seared on all sides before removing.
Combine the mustard, thyme, and horseradish in a bowl.
Spread the mustard mix over the beef evenly.
Place the beef back into the fridge uncovered.
If your puff pastry is in multiple pieces, use the egg wash to combine them. Be sure to place the pastry onto a well floured surface.
Use a roller to ensure the pastry is fully sealed and one piece. Roll the pastry as thin as needed to ensure it will fully wrap around the beef. About 1/4" thick is ideal though.
Weave the ham and them spread the Duxelles over the ham.
Place the tenderloin on top of the Duxelles.
Wrap the beef up like a baby, using the egg wash to help seal it.
Take any extra pastry you have remaining and decorate. I used a shamrock cookie cutter in this instance. Cover entire surface in remaining egg wash.
Cut slits through the top of the pastry. These are not only decorative but also help vent the beef. Sprinkle with the salt.
Place into your oven (or in this case your kamado / Big Green Egg) at 425 degrees until your desired internal temp is met. About 1 hour give or take. If cooking in a kamado I'd suggest using a pizza stone and parchment paper.
Remove and let rest.
Serve & Enjoy! See my notes for the recipes that I paired my beef wellington with.