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Preheat oven to 400 degrees.
In a large saucepan, add cornstarch, sugar and salt and stir together. Add warm water, lemon zest, lemon juice and stir until well-combined. Cook over medium heat, stirring constantly until the mixture boils. Remove from heat. Set aside.
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In a small bowl, beat 4 egg yolks with a whisk and then slowly pour the egg mixture into the the sauce, stirring quickly to avoid making any lumps. Return to the heat and cook, stirring until the mixture is thick but NOT boiling.
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Add margarine (or butter) to thickened mixture and stir until melted and blended throughout the mixture. Pour mixture into piecrust and make the meringue topping.
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Place 4 egg whites and salt in bowl of standing mixer (or use handheld mixer) and beat at high speed until stiff peaks form. As peaks are getting stiff, start to sprinkle in the sugar, 2 tbsp. at a time. When peaks are glossy and stiff, they are ready. Use the back of a wooden spoon to spread the meringue over the pie filling, swirling it up to make attractive swirls and pointy tips.
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Bake for 10 minutes or until lightly golden brown. Cool before slicing.