1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). Spray a 9 x 13-inch (20- by 30-centimeter) baking pan with cooking spray.
2. For the crust, sift together flour and sugar. Mix in the margarine with a pastry cutter or by hand and work it in until it forms a dough.
3. Press the dough into the bottom of the prepared baking pan. You’ll have to keep pressing and spreading to get it to cover the entire surface of the pan. Bake for 20–25 minutes, until slightly golden.
4. While the crust is baking, prepare the filling: Whisk together the eggs, sugar, and flour. Stir in lemon and lime juices, non-dairy whipping cream, and salt.
5. When the crust is ready, remove it from the oven. Whisk the filling one more time and pour into the warm crust. Return the pan to the oven and bake for 25 minutes, until filling is set. When cool, sprinkle with additional confectioners’ sugar.