From America's Test Kitchen Season 2: Two French Tarts
Despite its apparent simplicity, there is much that can go wrong with a lemon tart. It can slip over the edge of sweet into cloying; its tartness can grab at your throat; it can be gluey or eggy or, even worse, metallic-tasting. Its crust can be too hard, too soft, too thick, or too sweet. We wanted a proper tart, one in which the filling is baked with the shell. For us, that meant only one thing: lemon curd.
For just enough sugar to offset the acid in the lemons, we used 3 parts sugar to 2 parts lemon juice, plus a whopping 1/4 cup of lemon zest. To achieve a curd that was creamy and dense with a vibrant lemony yellow color, we used a combination of whole eggs and egg yolks. We cooked the curd over direct heat, then whisked in cold butter. And for a smooth, light texture, we strained the curd, then stirred in heavy cream just before baking.
The filling is a lemon curd adapted for this tart. Once the lemon curd ingredients have been combined, cook the curd immediately; otherwise it will have a grainy finished texture. To prevent the curd from acquiring a metallic taste, make absolutely sure that all utensils coming into contact with it--bowls, whisk, saucepan, and strainer--are made of non- reactive stainless steel or glass. Since the tart pan has a removable bottom, it is more easily maneuvered when set on a cookie sheet. If your pre-baked tart shell has already cooled, place it in the oven just before you start the curd and heat it until warm, about 5 minutes. Serve the tart with lightly whipped cream, the perfect accompaniment to the rich, intensely lemon filling.