Preheat oven to 350°F. Coat a 9-inch tart pan with removable bottom with nonstick cooking spray.
To prepare the Ginger-Oat Crust: In a medium bowl, beat softened butter with an electric mixer on medium to high speed for 30 seconds. Add packed brown sugar or brown sugar substitute equivalent to 2 tablespoons brown sugar. Beat until combined, scraping bowl occasionally. Beat in egg white and vanilla until combined. Beat in all-purpose flour. Stir in ground gingersnaps, quick-cooking rolled oats, and ground ginger.
Using your damp fingers, press crust dough onto bottom and up the side of the prepared tart pan. Line crust with a double thickness of foil that has been lightly coated on the bottom with nonstick cooking spray. Bake for 8 minutes. Carefully remove foil. Bake for 5 to 7 minutes more or until crust is lightly browned. Cool completely on a wire rack.
To prepare the Lemon Filling: In a small bowl, use a fork to lightly beat egg yolks; set aside. In a medium saucepan, stir together sugar and cornstarch. Stir in lemon peel, lemon juice, and the water. Cook and stir over medium heat until thickened and bubbly. Stir half of the lemon mixture into egg yolks. Add egg mixture to lemon mixture in saucepan. Cook and stir over medium heat until mixture comes to a gentle boil. Cook and stir for 2 minutes more. Remove from heat. Whisk in vegetable oil spread until well mixed. Cover surface of the cooked lemon mixture with plastic wrap. Let stand at room temperature for 15 minutes. Whisk in sour cream until well mixed.
Pour lemon mixture into cooled tart crust. Cover loosely with plastic wrap or foil and chill at least 1 hour before serving. Remove side of pan to serve. If desired, top with lemon, lime, and/or orange slices.
Tip: If using sugar substitutes, choose Splenda® Blend for Baking to substitute for the ⅔ cup sugar. Choose from Sweet 'N Low® Brown or Sugar Twin® Granulated Brown to substitute for the brown sugar. Follow package directions to use product amounts equivalent to ⅔ cup sugar or 2 tablespoons brown sugar. Nutrition Facts per serving with substitute: same as below, except 192 cal., 20 g carb., 120 mg sodium. Exchanges: 1½ other carb. Carb Choice: 1