Heat oven to 250°F. Line cookie sheet with parchment paper. Draw six 4-inch circles on paper.
In large bowl, beat egg whites and cream of tartar at medium speed until soft peaks form. Gradually add 1/2 cup sugar, beating at high speed until stiff glossy peaks form and sugar is almost dissolved. Spoon meringue onto circles on paper-lined cookie sheet, building up edges to form 1-inch-high sides.
Bake at 250°F. for 1 hour. DO NOT OPEN OVEN. Turn oven off; leave meringues in oven with door closed for 2 hours.
Meanwhile, in medium saucepan, combine 1/4 cup sugar and cornstarch. Slowly stir in orange juice. Cook and stir over medium-low heat until mixture thickens. Stir in lemon juice and lemon peel. Cool completely. Store filling in refrigerator.
To serve, spoon lemon filling into meringue shells. Sprinkle with powdered sugar. If desired, garnish with mint leaves.