Step 1Combine the flour, sugar, salt and butter in a food processor or large bowl; cut them together until the mixture resembles coarse cornmeal. Add the water 1 tablespoon at a time, stirring constantly or processing until the dough 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. Press the overhanging dough against the edges of the pan to cut away the excess. 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 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, an additional 12 to 15 minutes. Remove the crust from the oven and cool to room temperature.
Step 1Put a small bowl in the refrigerator to chill. (You will use it later to cool the hot lemon curd.) In a small saucepan, beat the eggs, yolks, salt and sugar until smooth and light-colored.
Step 2Add the lemon juice, zest of 1 Meyer lemon and butter and cook over medium heat, stirring constantly, until the 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 the consistency of thick hollandaise sauce. Pour it through a fine strainer into a chilled bowl and let stand to cool to room temperature.
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.
Step 5While the tart is chilling, blanch the zest of 1 lemon in boiling water just enough to soften it slightly, about 15 seconds. Drain and combine it in a small saucepan with the grenadine. Bring to a simmer and cook until the zest has colored, about 15 minutes. Drain, empty onto a plate, separate and allow to dry about 30 minutes. Arrange the zest over the center of the tart before serving.