Preheat your oven to 410ºF/200ºC.
Wash and pat dry the celeriac, do not peel it; rub it with olive oil, salt, and pepper, wrap the celeriac in aluminum foil and bake it for 1:40 minutes.
Remove the celeriac from the oven and let it cool, when it is at room temp, slice it into medium-thick slices (about half centimeter).
Over medium heat, add olive oil to a pot, when hot, add the mixed mushrooms, a pinch of salt and sauté the mushrooms until they have browned, this will ensure your mushrooms are tasty and rich instead of blend and soggy.
Remove the mushrooms from the pot, add vegan butter to it, then add the sliced leeks and a pinch of salt, the herbs de Provence and stir until the leeks are translucent and soft.
Add the nutmeg, stir and add the flour, stir for 5 minutes to cook the flour, then add the UNSWEETENED plant-based milk, black pepper, and nutritional yeast, add the mushrooms to the pot and stir well to combine. Lower the heat and let it cook for 7-10 minutes, stirring occasionally.
When the mushroom leeks and bechamel is ready, allow it cool and reach room temp.
Take an 8-inch diameter X 3.5 inches deep bowl, cover with plastic wrap, and line the sliced celeriac in and around the bowl. Then, add a layer of the mushroom leeks bechamel, then another layer of the roasted celeriac, and so on until you complete the bowl, finishing it with a layer of roasted celeriac.
Place another sheet of plastic film on top of your pie, add a plate to the top then place some weight on top of the plate, we need the pie compact and firm and for this, we are going to place this bowl in the fridge for at least 4 hours, ideally 6 hours or overnight.
Preheat oven to 410ºF/200ºC.
Add a sheet of puff pastry to the bottom of your baking sheet, the pastry dough should be bigger than the bowl by at least 1,5 inches.
Prick it all over with a fork so it doesn’t rise.
Remove the bowl from the fridge, remove the top plastic film, place it in the center of the pastry dough, remove the other plastic film and place the puff pastry on top of the dome, adjusting it tight so it doesn’t create pockets of air and pinch the borders of the top puf pastry along with the bottom puff pastry tightly yet gently using three fingers. This will seal the pithivier and prevent the bechamel from leaking to the baking sheet while baking.
Make the design you like to the dome of your pithivier, make a small hole on the top of it so the puff pastry doesn’t inflate, brush it with the ketchup, mayo, and butter mixture, put it in the freezer for 15 minutes, then bake the pithivier for 35/40 min. If using a convection oven, bake at 375ºF/190ºC. The internal temperature of the pithivier should not go over 55ºC/131ºF. Let the pithivier rest 20 minutes prior to slicing
Serve with a brown gravy or bread sauce or both and bon appétit!