In the bowl of a food processor combine the sugar, flour, butter and salt until the mixture resembles breadcrumbs. With the motor running add the beaten egg. When the mixture forms a ball, form a disk, place it in a plastic bag and refrigerate for 1 hour. Preheat the oven to 425F and grease a 10-inch loose-bottom tart pan. Roll out the dough to a 1/4-inch thick and line the tart pan. Place a piece of foil in the tart pan and fill it with dry beans or rice. Bake for 25 minutes. Remove the foil and beans or rice and cool on a rack. To make the filling, whisk together 2 of the eggs and 1/2 cup of the sugar. Stir in the vanilla and the flour and mix well. Beat in the remaining egg. Scald the milk and salt over a moderate heat and pour the milk into the egg mixture, beating constantly. Return the egg mixture to the saucepan where the milk was scalded and cook gently, stirring, until thickened. Remove the pan from the heat and stir in the Grand Marnier. Thinly grate the zest of 1 of the oranges and cook in boiling water for 1 minute. Drain and rinse in cold water. Add the zest to the egg mixture. Pour the egg mixture into the pie crust. Thinly slice the remaining oranges and place them on top of the cream.
