How to Make It
In a medium heatproof bowl, whisk the egg yolks with 1 cup of the sugar, the lemon juice and lemon zest. Fill a medium saucepan with 1 inch of water and bring to a boil. Set the bowl over the boiling water, reduce the heat to moderate and cook, whisking the lemon mixture constantly, until thickened, about 12 minutes. Remove the bowl from the heat and whisk in the cubed butter. Fill the crust with the warm lemon curd and refrigerate until the curd is firm enough to cut, about 3 hours.
Using a sharp knife, peel the grapefruit, lemon and lime, removing the bitter white pith. Cut in between the membranes to release the citrus sections. In a medium skillet, melt the remaining stick of butter. Add the pecans and cook over moderate heat, stirring, until golden, about 6 minutes. Remove the skillet from the heat and add the Grand Marnier. Return the skillet to the heat and carefully ignite the liquid with a long match. When the flames subside, add the sugar and water mixture and boil until thickened, about 1 minute. Add the cream and salt and boil for 1 minute.
Just before serving, stir the fruit sections into the sauce. Cut the tart into wedges and serve with the sauce.