I based this recipe on one that Nigel Slater wrote up last November, but I wanted to add more vegetables, and I didn't have any crème fraiche. As such, the filling in my take on his recipe has little in common with Nigel's original. Despite being vegetarian, this is definitely more of a "comfort food" than it is "healthy."
OVERVIEW
Prep - Quick, less than 10 minutes
Cook - About 90 minutes, in stages.
Serves - 4-5 as a main dish
You'll need - Leeks, Onions, Mushrooms, Parmesan, Soft Cheese, Cream, Flour, Eggs, Butter
RECIPE
Start with the crust - measure out 180g of butter (about a stick and a half) from the fridge, then dice it into cubes about a quarter inch to a side. Add in 300g of flour (one and a quarter cups) and (with clean hands) squish the chunks of butter into the flour, until all your little butter cubes have been squashed/mixed into a semi-consistent "dry slurry" of flour and butter. Drop in an egg yolk and about 50ml of water (about a quarter cup) . Mix that up with your hands until it looks like dough, then ball it up and stick it in the fridge for a bit.
While the dough chills, slice about 2 pounds of leeks into half-inch rounds, then rinse those to get rid of any sand. Leeks are delicious, but they've got that unfortunate trick of hiding sand between their layers, like an onion that spent too long at the beach. Once cleaned, toss the leeks into a heavy pan with more butter (half a stick) as well as salt and pepper, cover them, and cook them on low heat for a solid 20 minutes, stirring once or twice as you go. You don't want much color on the leeks; cooking them gently keeps their flavor mild.
While the leeks cook down, grab about 15 button mushrooms, clean and quarter them. Slice 4 cloves of garlic thinly. While you're at it, slice up an onion into thin strips. You can probably put the cutting board away after that.
The leeks will probably be about done when you finish, so tip those into a bowl and replace them with the mushrooms and garlic. Those won't need a lid, just stir them occasionally for 10-15 minutes over low/medium heat while you return to the crust. Add salt and pepper; you probably don't need more butter - the leaks will have left a decent sheen on the pan - but keep an eye on the garlic so that it doesn't burn. As the mushrooms cook down, they'll (ideally) leak moisture into the pan so the garlic doesn't dry out, but such things are fickle.
This is a good time to preheat the oven to 395f (or 200c if you can use the word "chuffed" in a sentence). Toss a large baking sheet in there to heat along with the oven, and clear a (lightly floured) space on your counter to roll out your dough.
Working slowly, flatten the chilled dough-ball on the counter with a rolling pin, enjoying the wonderful smells around you. Roll it out to a thin pie crust, probably a scant 1/4 inch. You want it to be a bit bigger than a dinner plate. Once it's there, transfer it to a sheet of parchment paper, but leave it on the counter for now.
The mushrooms & garlic should be done around the same time as your crust, so transfer them to the bowl of leeks, add a bit of oil to the pan (if needed) and then cook the onions. Again, low/medium heat will work just fine.
While the onions caramelize, prep the cheese. Start with parmesan - you're looking for 80g of parmesan (or 3 ounces) as well as about 140g of a soft and creamy cheese (5 ounces) - I used a package of Boursin since it was getting old, but goat cheese would work great here too. Grate the parmesan, and break the soft cheese up into smallish chunks.
The onions probably still need time, so give them a stir and prep your egg wash by cracking an egg into a small bowl, whisking it, and tracking down a pastry brush.
When the onions are ready (I cook the tar out of onions, since I prefer them to be a bit noodlish) you can add all the other vegetables to the pan. Add in a sprinkling of flour to help it thicken. Stir that all together, add in the cheeses, and pour in about half a pint of heavy cream (about a cup). Let all of that melt together for a bit, stirring frequently, until the cream is absorbed.
Dump the filling to the center of your pie crust, then fold the crust partway up around it, working around the sides and leaving the center open. Brush the crust with your egg wash, then carefully transfer the pie (You can use the parchment paper to "slide" the pie) onto your preheated baking sheet.
Let the pie cook for about 40 minutes, then allow 10 more for it to cool. Enjoy!