You could always use a package of lemon pie filling, but there is nothing so tart and lemony as a filling you made yourself out of fresh lemon juice.
1 cup sugar
3 tablespoons cornstarch
1 ½ cups cold water
3 egg yolks, slightly beaten
grated rind of one lemon
¼ cup lemon juice
1 tablespoon corn oil margarine
1 baked 9 inch pastry shell*
3 egg whites
1/3 cup sugar
In a 2 quart saucepan, stir together 1 cup sugar and cornstarch. Gradually stir in water until smooth. Stir in egg yolks. Stirring constantly, bring to a boil over medium heat and boil for 1 min.; remove from heat. Stir in lemon rind, lemon juice, and margarine. Place in a large bowl, and covers surface with plastic wrap. Cool slightly (no more than 1 hour).
Spoon filling into pastry shell.
Place egg whites in a small bowl, and beat at high speed until foamy. Add 1/3 cup sugar, 1 tablespoon of the time, beating well after each addition. Continue beating until stiff peaks form. Spread some of the meringue around the edge of the filling first, so that it touches the crust all the way around, then fill in the center.
Bake at 350° 15-20 min., or until lightly browned. Cool at room temperature, away from drafts.
*Click on this link in order to find Alton Brown’s crystal-clear directions on how to do this correctly. The procedure is not all that complicated, but I just like Alton Brown.