There are places aplenty to get cornmeal, but I suggest seeking out something rare and exceedingly flavorful for this crust. Anson Mills and Louismill both have amazing cornmeal, but for this crust, I like Community Grains Flint cornmeal. Find it here.
To make the filling, stir the pumpkin puree and sour cream together, and place in a colander lined with butter muslin or cheesecloth. Place the colander over a bowl to let the excess water slowly seep out for the next hour or so.
In the meantime, make the crust. You might already know this, but it helps to get everything good and cold when making pastry crust. If I think of it, I’ll even put the flour in the freezer the night before.
At any rate, mix the flour, cornmeal, salt, and sugar together. Set aside (in the fridge or freezer if possible). Cut up the butter into manageable cubes and tuck back into the fridge until you’re all set to go. Crack the egg and beat it lightly. Add two tablespoons of cold milk. Have a little more cold milk handy in case your dough isn’t coming together and you need to add another tablespoon. Now, bring out your flour mixture and cold butter, and using a pastry blender or two knives or your fingertips, cut the butter into the flour until it looks like a pile of coarsely grated parmesan cheese. Quickly add your egg mixture and stir with a wooden spoon until it just starts to hold together. Here’s where you might need to add another tablespoon of cold milk. When it just starts to come together, gather the dough into a heap and plop it onto a big piece of parchment paper or plastic wrap. Shape it roughly into a square, then wrap it up and tuck it away in the fridge so it can chill out and relax. Leave it there at least an hour, but as long as three days, if needed.
Preheat the oven to 375°F.
Now finish making your filling. Scrape the pumpkin mixture into a bowl and add your eggs, brown sugar, cinnamon, nutmeg, ginger, and salt. Stir to combine. Set aside until your dough is ready.
When you’re ready to bake, roll out the dough to roughly 20×15 inches. Lay it in a 9×13-inch baking pan or equivalent oval baking dish. The edges will hang over the sides at this point, but just merrily ignore that and spread the filling into the bottom of the dough and spread to an even layer. Fold the dough back over the filling in kind of haphazard fashion. No need to be fussy here, just fling the dough back onto itself and know that it will get all cozy and comfy on its own. You can even tear pieces off where it’s hanging too low and add them to places where it’s not large enough. If you completely cover the filling with dough, make a few vent holes, but sometimes these just naturally show up on their own. Brush with a little cream and get that baby in the oven. Bake for about 45 minutes until the crust is puffed and golden, and the filling is set and jiggling a little. Remove from the oven and cool on a wire rack for about an hour. Slice and savor with delight at having ease on the Thanksgiving table and gratitude for sharing it with those you love.