caramelized onion and mushroom tart

caramelized onion and mushroom tart, originally uploaded by riptideredsf.

Two years ago when I started educating myself about cooking, I knew nothing about caramelized onions. I didn't know what "caramelized" even meant. Sure, I'd cooked onions until they were brown, blindly following recipes without any analyzation of the process or result. Sugar in onions? Who knew? A few french onion soups had been produced throughout a lifetime of sporadic cooking, rarely processing the process. It was a  means to an end, satisfying hunger as quickly as possible, or occasional entertainment, and as long as the result wasn't burnt or raw or oversalted and tasted good (which meant edible) , the task was deemed a success, tagged with a bookmark or stuffed into a binder and I moved on.

I discovered a recipe for Onion Jam  in Kim Boyce's Good to the Grain last year. It was only then that  I fully understood the process. Suddenly, all of that browning and caramelizaton talk from Americas Test Kitchen finally made sense. If I had to choose my biggest kitchen AHA! moment, that would be it.
It takes about 90 minutes to cook the onions down their jammy consistency. Onion jam on a BLT is the most amazing thing ever. I mixed some into a tuna salad today that was off-the-hook. Even on a fork, all by itself, it's the ultimate marraige of sweet and savory in one bite. Caramelization is a beautiful thing and shouldn't be rushed. Onions for this tart don't have to be cooked this long though, you want more structure.

I came across this article at Slate.com this morning that was dead-on, humourous and an eye-opener about writing recipes. The writer accuses recipe writers of lying about how long it takes to caramelize onions, because the long cooking time is a deterrent to a recipes appeal. 

The article concludes: "The best time to caramelize onions is yesterday".

Earlier in the week I caramelized about 3 pounds of onions in a dutch oven. I was left with about 2 cups of gooey good onions. It keeps in an airtight jar for a week, though rarely lasts that long. I use caramelized onions in all kinds of ways, the breakfast tacodilla is it's most frequent destiny.

 I've been in  pastry mode for a couple of weeks now, so when I scored  giant bag of button mushrooms from the Chinese market I knew a tart was in my immediate future. This is a hybrid of recipes adapted from Smitten Kitchen and Dorie Greenspan's Around my French Table.

caramelized onions-again, way more than you need for this recipe and do this ahead of time. A dutch oven is great when cooking a large batch because the cast iron has great heat distribution, there's a lot of surface area to promote browning.

2 tablespoons oil
3 large yellow onions (sliced 1/4 thick)
1 teaspoon salt
black pepper
1 teaspoon red wine vinegar

Heat the oil in dutch oven on medium-high heat and then add the onions. Toss until the onions are coated and cook until they start to brown, stirring infrequently, just to redistribute the onions so they come in contact with the surface of the pot. Once the browning starts (it take's about 20 minutes to get there), add salt, lower the heat and cover the pot and cook for another 20 minutes so the onions release their moisture and get really soft.

Remove the cover and continue to cook for another 30 minutes until the water evaporates and the onions become brown. De-glaze with red wine vinegar at the end.

Pastry
from smitten kitchen

1 1/4 cups flour
1 tablespoon plus 2 teaspoons cornstarch
1/4 teaspoon salt
6 tablespoons butter, diced
1 egg

In the food processor pulse the dry ingredients to combine. Pulse butter until it's in bits the size of small peas. Pulse in the egg until the dough forms.

On a floured work surface roll the dough into a 12 inch circle, carefully roll it over the rolling pin to lift it off the counter and into the tart pan. Press the dough into the corner of the pan and trim off the excess. If you're using a fluted tart pan with a removable base lift the tart pan by the edge and put it onto a plate. Cover with plastic wrap and chill for 30 minutes.

filling:

2 tablespoons butter
1 pound white button mushrooms-stems removed, sliced
1/2 teaspoon salt
1 teaspoon dried tarragon
1 cup of beef stock ( or vegetable stock)
1 tablespoon fresh thyme
1/2 cup caramelized onions
2 eggs
1/2 cup heavy cream
1/2 cup fontina cheese (or gruyere)
salt
pepper
nutmeg
1 teaspoon dijon mustard

Preheat the oven to 350 degrees.

Remove the chilled tart shell from the fridge. Line with foil or parchment and weight the pastry down with pie weights, dried beans or rice. I keep a jar of white beans labeled For Pastry and re-use.

Bake for 10 minutes, remove the weights, foil or parchment and bake for another 5 minutes, until the pastry appears dry, but not browning.

Remove from the oven and brush with dijon mustard. Set aside

While the pastry is par-baking (also called blind baking) heat up a  large skillet over medium high heat, melt the butter until it stops foaming. Add the sliced mushrooms and toss to coat, then let them cook undisturbed, about 5-7 minutes until they start to brown. Toss again to redistribute and continue to brown. Add the salt, pepper and tarragon. The mushrooms will release their water. Cook uncovered until the water evaporates and a fond  (the browned bits) forms on the skillet. Add the beef broth and de-glaze the skillet. Continue to cook until the broth reduces. Remove from the heat and stir in fresh thyme.

In a small bowl or a large pyrex measuring cup, wisk the eggs, cream, salt, pepper and nutmeg together then stir in the grated cheese.

Place your par-baked tart shell on a sheet pan, spoon in the caramelized onions and spread it evenly over the bottom. Then add your mushroom layer spreading it evenly over the onions. Pour the egg mixture over the surface.

Place the sheet pan on th middle rack of the oven and bake for 35-40 minutes until the center is puffed and brown. Let it rest for about 10 minutes, remove the fluted ring from the base. Slice and enjoy with a big green salad.

This tart re-heats nicely and I pack it for lunch.