In a large dutch oven, combine the butter, dried thyme, coriander, salt and pepper. Place on the stove and melt butter over low heat while you chop your veggies.
Once the butter is melted, add the leeks, sweet potatoes, parsnips, carrots and celery root and give the mixture a couple stirs to coat the veggies in butter.
Transfer the Dutch oven to the oven. Roast the veggies for 30 minutes, or until slightly tender. Make sure to stir the mixture every 10 minutes for even cooking. While the veggies are cooking, prep the rest of your ingredients.
When the veggies are slightly tender, transfer the pot back to the stove and turn the oven down to 400F.
Over medium heat, deglaze the pan with the cognac. Once the cognac has lost it's alcohol smell, add the flour and cook for one minute, stirring constantly.
Pour in the stock and milk and mix well, being careful to not crush the root vegetables.
Bring to a boil. The sauce should be nice and thick at this point.
Turn off the heat and stir in the herbs.
Reseason with salt and pepper to taste.
If desired, transfer the filling to individual, heat safe bowls.
Roll out a sheet of puff pastry according to the package instructions. Cut squares large enough to drape over your pies and then make a few slits in the top of each to vent the steam.
Place the pastry over your pies and then brush with egg wash.
Bake pot pies for 25-30 minutes, or until the puff pastry is golden brown and the filling is bubbling.
You can make the filling in advance, so this is a great recipe for meal prep. Just cover with puff pastry and bake. They will take a bit longer to bake if the filling is cold.
There's roughly a half pound of each vegetable in this recipe.
We tested this recipe with Pepperidge Farm and Wegmans brand puff pastry. The Wegmans puff is made with real butter, so we found it more tasty but easier to burn in the oven.