Step 1Combine the flour, sugar, salt and butter in a food processor or large bowl and cut them together until the mixture resembles coarse cornmeal. Add the water 1 tablespoon at a time, stirring constantly or processing until the mixture just begins to come together.
Step 2Remove the dough from the bowl and knead it lightly and briefly to make a smooth mass. Wrap it in plastic wrap and refrigerate for at least 30 minutes.
Step 3On a well-floured work surface, roll out the dough into a circle about 11 inches in diameter. Roll the dough back onto the rolling pin and transfer it to a buttered 9-inch tart pan with a removable bottom. Unroll the dough and gently press it into the pan. Trim the excess dough to 1 inch from the pan edges and fold the extra dough over itself between the pan and the dough rim to make a sturdier, taller edge. Refrigerate for 30 minutes.
Step 4Heat the oven to 425 degrees. Prick the crust with a fork. Line the crust with a sheet of aluminum foil and fill it with rice, dried beans or pie weights. Bake the crust 10 minutes. Remove the foil and beans and bake until the crust is golden brown and firm, another 15 minutes.
Step 5Remove the crust from the oven and cool to room temperature.
Step 1Beat the eggs, yolks, salt and sugar in a small saucepan until smooth and light colored.
Step 2Add the lemon juice, lemon zest and butter and cook over medium heat, stirring constantly, until butter melts, about 2 minutes.
Step 3Reduce the heat to medium-low and continue cooking and stirring until the curd is thick enough that it coats the back of the spoon and when you draw your finger across the curd it leaves a definite track, about 5 minutes. The curd should be as thick as thick hollandaise. Pour it through a fine strainer into a chilled bowl.
Step 4Spoon the curd into the prepared crust and smooth the top with the back of a spoon. Refrigerate the tart for at least an hour to set the lemon curd.