Lemon-Lime Tart for Sweetie

 Valentine's Day is always a bit of a challenge, coming as it does the day after my birthday. This year Sweetie requested a pie or tart for Valentine's Day and I had already decided to make the Queen Mother Cake for my birthday.

I bought a bag of good-sized limes, thinking that I would make a lime tart, but then we received a gift of a bag of beautiful lemons from a neighbor, so I decided to go for a lemon-lime tart for Valentine's Day.

My favorite lemon tart filling is Dorie Greenspan's, so I began with that recipe and added lime zest and juice to the lemon zest and juice, with a decoration, after the tart was finished, of strands of zest from both fruits. The tart case is fairly simple and comes out cookie-like so that the filling doesn't make it too gooey after a day. We had the last slice, shared, two days after I filled the tart shell and the base was still crisp.

As it turned out, we had the Queen Mother cake on Valentine's Day, right after Sweetie's fire board meeting. Then we had the Lemon-Lime Tart the next day and were able to share it with our daughter and her fiancé'. I was worth the wait and well worth your time to make. It would be perfect for St. Patrick's Day, which is coming up soon. It would also be delightful for Easter...or anytime, really.

The Most Extraordinary French Lemon - Lime Cream Tart 

Based on a recipe by Dorie Greenspan in Baking, From My Home To Yours

1 cup sugar
Grated zest of 2 lemons
Grated zest of 2 limes, (or 3 if smaller limes)
4 large eggs at room temperature

3/4 cup fresh lemon juice mixed with lime juice (from 3-4 large lemons or up to 6 smaller ones, plus2-4 limes)
2 sticks plus 5 tablespoons (10 1/2 ounces) unsalted butter, cut into tablespoon-sized pieces, at room temperature 
1 9-inch tart shell made with Sweet Tart Dough (recipe follows), fully baked and cooled

Have an instant-read thermometer, a strainer (if straining out the zest), and a blender (first choice) or food processor at hand.

Directions:
Bring a few inches of water to simmer in a saucepan or the bottom of a double boiler.

Put the sugar and zest in a large heatproof bowl that can be set over the pan of simmering water, or in the top pan of a double boiler. Off the heat, rub the sugar and zest together between your fingers until the sugar is moist, grainy and very aromatic. Whisk in the eggs, followed by the lemon-lime juice.

Set the bowl over the pan, or set the top into the bottom of the double boiler, and start stirring with the whisk as soon as the mixture feels tepid to the touch. Cook the lemon cream until it reaches 180 degrees F. As you whisk and you must whisk constantly to keep the eggs from scrambling, you'll see that the cream will start out light and foamy, then the bubbles will get bigger, and then as it gets closer to 180 degrees F, it will start to thicken and the whisk will leave tracks, which means that the cream is almost ready. Don't stop whisking! Continue to check the temperature. It might take a while. so be patient. Usually it's done in about 10 minutes.

As soon as the cream reaches 180 degrees F, remove from the heat and, if removing the zest, strain it into the container of the blender (or food processor); discard the zest. (If not removing the zest, just scrape the cream right into the blender or food processor). Let the cream stand, stirring occasionally, until it cools to 140 degrees F, about 10 minutes.

Turn the blender to high (or turn on the food processor) and, with the machine going, add the butter, about 5 pieces at a time. Scrape down the sided as needed as you incorporate the butter. Once all the butter is in, keep blending/processing for another 3 minutes.

Pour the cream into a container (I used a large Pyrex bowl), press a piece of plastic wrap against the surface to create an airtight seal and refrigerate at least 4 hours or overnight.

When you are ready to assemble the tart, just whisk the cream to loosen it and spoon it into the tart shell. I like to swirl the top with the back of a spoon. If desired, decorate the top with strands of lemon and lime zest for additional lemons and limes. Serve the tart, or refrigerate until needed.

Note: The filling will keep in the fridge for 4 days, or tightly sealed, in the freezer for up to 2 months. Thaw it overnight in the refrigerator and whisk before putting into the tart shell.

Sweet Tart Dough
From Dorie Greenspan

1 1/2 cups all-purpose flour
1/2 cup confectioners' sugar
1/4 teaspoon salt
1 stick plus 1 tablespoon (9 tablespoons) very cold (Frozen) unsalted butter, cut into small pieces
1 large egg yolk

Put the flour, confectioners' sugar and salt into a food processor and pulse a couple of times to combine. Scatter the pieces of butter over the dry ingredients and pulse until the butter is coarsely cut in- there will be some tiny pieces and some the size of peas.

Stir the yolk to break it up, then add it a little at a time, pulsing afer each addition. When the egg is in, process in long pulses - about 10 seconds each - until the dough forms clumps and curds. Turn the dough out onto a work surface and, knead lightly just to incorporate any dry ingredients that might have escaped mixing. In all of this, don't overwork the dough.

Butter a 9-inch tart pan with a removable bottom. Press clumps of the dough evenly over the bottom and up the sides of the pan, using all but one little piece (about a teaspoon worth), which you should save in the fridge wrapped in plastic wrap to patch any cracks after the crust is baked. Freeze the crust for at least 30 minutes, preferably longer, before baking.

Center a rack in the oven and preheat to 375 degrees F.

Butter the shiny side of a piece of aluminum foil and fit the foil, buttered side down, tightly against the crust. Put the tart pan on a baking sheet and bake the crust for 25 minutes. Carefully remove the foil. If the crust ahs puffed, press it down gently with the back of a spoon. Patch the cruse if necessary, then bake for another 8 minutes or so, until it is firm and golden brown. Keep an eye on it the last few minutes and pull it out if it gets darker than golden. Transfer the pan to a cooling rack and cool to room temperature before filling.