1/2 cup cornstarch
2 1/2 teaspoons lemon zest plus 6 tablespoons fresh lemon juice (from 3 lemons), divided
1/4 cup fresh lime juice (from 2 limes)
1 2/3 cups water
2 1/2 cups granulated sugar, divided
2 tablespoons unsalted butter, softened
1/4 teaspoon fine sea salt, divided
4 large eggs, separated
Prebaked Orange Pastry Shell
1/4 teaspoon cream of tartar
Preheat oven to 350°F. Stir cornstarch, lemon juice, and lime juice in a medium non-reactive saucepan until cornstarch dissolves. Stir in 1 1/2 cups water, 1 1/2 cups sugar, butter, 1/8 teaspoon salt, and lemon zest. Bring to a boil over medium-low, stirring constantly with a wooden spoon until thickened, about 15 minutes. (Mixture will thicken as it cooks.) Remove from heat.
Whisk egg yolks in a large bowl. Gradually whisk in hot lemon-lime mixture. Pour mixture into pastry shell. Press a piece of plastic wrap directly onto the surface and let cool for 30 minutes. Remove plastic wrap.
Combine egg whites and the remaining 1/8 teaspoon salt in the bowl of a heavy-duty stand mixer fitted with the whisk attachment. Beat on medium speed until frothy, about 1 minute. Gradually add cream of tartar and remaining 1/2 cup sugar, beating on medium speed until mixture forms stiff, glossy peaks, about 4 minutes.
Gently dollop meringue over warm lemon-lime filling. Using the back of a spoon, spread meringue to the edges. Swirl decoratively with a spoon. Broil in preheated oven until meringue is golden brown, about 1 to 2 minutes. Transfer to a wire rack, and let cool to room temperature for about 1 hour. Transfer to refrigerator until well chilled, about 3 hours.
