This tart has three main elements: the crust, the curd, and the meringue. Both the crust dough and the curd can be made in advance and then assembled, but the meringue must be made the day the tart is served. Meringue can be piped on the tart or spooned on in easy swirls. Or not at all! This tart is also delicious without the meringue.The lemon curd is based on a recipe for “The Most Extraordinary Lemon Tart”by the great Dori Greenspan. She calls the curd “lemon cream”; it can be refrigerated for four days and frozen for up to two months prior to assembling in the tart crust.
Blend the flour, sugar, and almonds in a food processor until the nuts are finely ground. Using on/off turns, cut in the butter until a coarse meal forms. Add the egg and blend just until dough forms. Gather dough into ball; flatten into a square 1-inch thick, wrap in plastic, and chill 1 hour.
Line the bottom of a 10-inch round, removable bottom tart pan with parchment paper. Cut the dough into1-inch slices.Lay the slices in the bottom of the pan and push them together, closing all fissures. Press the bottom of a glass against the dough to flatten and smooth. Then line the edges of the tart with slices of dough placed horizontally around the fluted edges. Press this dough into the bottom of the crust and into the fluted rim. Refrigerate the crust for one hour.
Preheat oven to 375°F. Bake the crust until golden brown, about 17 minutes. Remove the crust from the oven. You will notice that the crust is somewhat puffed up; this should be tamped down with the bottom of a glass or similar flat, heavy tool. Cool the crust and then remove the fluted ring by setting the tart pan over a jar and letting the ring fall to the counter. Use a flat metal spatula to lift the tart off of the metal tart pan bottom. Place on a plate to fill with curd and top with meringue.
Have a thermometer, preferably an instant-read, a strainer,and a blender (first choice) or food processor at the ready. Bring a few inches of water to a simmer in a saucepan.
Put the sugar and zest in a large metal bowl that can be fittedinto the pan of simmering water. Off heat, work 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 juice.
Fit the bowl into the pan (make certain the water doesn’t touch the bottom of the bowl) and cook, stirring with the whisk as soon as the mixture feels tepid to the touch. You want to cook the cream until it reaches 180°F. As you whisk the cream over heat—whisking 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 the cream is getting closer to 180°F, it will start to thicken and the whisk will leave tracks. Heads up at this point—the tracks mean the cream is almost ready. Don’t stop whisking and don’t stop checking the temperature. Getting to temp can take as long as 10 minutes.
As soon as you reach 180°F, pull the cream from theheat and strain it into the container of a blender (or food processor); discard the zest. Let the cream rest, stirring occasionally, for about 10 minutes.
Turn the blender to high and, with the machine going, add about 5 pieces of butter at a time. Turn off the blender and scrape down the sides of the container as needed while you’re incorporating the butter. Once the butter is in, keep the machine going—to get the perfect light, airy texture, you must continue to beat the cream for another 3 minutes. If your machine gets hot, give it a little rest between beats.
Pour the cream into a container, press a piece of plastic wrap against the surface to create an airtight seal,and chill the cream for at least 4 hours or overnight. When you are ready to construct the tart, just whisk the cream to loosen it and spoon it into the tart shell.
Make the meringue the same day you're serving the tart. In the bowl of a stand mixer, stir together the egg whites, sugar, and cream of tartar. I use the whisk attachment to stir. Set the bowl over a saucepan of simmering water, making sure that the bowl has some clearance above the water (we're cooking very gently with the residual or steam heat here). Heat, stirring frequently, until the sugar is dissolved and mixture reaches 175 degrees, about 8 minutes.
Transfer the whites to the stand mixer with the wire whisk attached. Beat, starting slow and increasing the speed steadily, until the mixer is on full. Whip until the stiff, glossy peaks form, 5 to 7 minutes.
Use a large, 1/2-inch piping tip to pipe the meringue on the tart, or spoon the meringue making decorative swirls on top.
Brown the meringue either by skimming the edges of the meringue with a kitchen torch, or by placing the tart in a 350 degree oven just until the meringue is lightly browned. Chill until the tart is cold, at least one hour. Sift powdered sugar around the edge of the tart, and serve chilled.