Makes approximately 1 1/2 cups.
Let your lemons warm to room temperature because they will release their juice more easily. Grate your lemon zest before squeezing the lemons for juice and just before using because the zest will dry out if it sits too long. After squeezing, strain the juice to remove any pulp. Grate lemons just before using as the zest will lose moisture if it sits too long.
In a stainless steel heavy bottomed pan (or in a bowl over simmering water), whisk together the eggs, sugar, and lemon juice until blended. Cook over medium heat, stirring constantly to prevent curdling. Cook until the mixture becomes thick like sour cream. It will be 160 degrees F. It takes about 10 minutes.
Remove from heat and immediately pour through a fine strainer to remove any lumps. Cut the butter into small pieces and whisk into the mixture until the butter has melted. Add the lemon zest and let cool.
The lemon curd will continue to thicken as it cools. Cover immediately with plastic wrap touching the entire surface of the curd so a skin doesn't form and refrigerate for up to a week.
The Meringue Topping
Bring the eggs whites to room temperature. In the bowl of your electric mixer with the whip attachment, beat the egg whites until frothy at medium speed. Add the cream of tartar. Beat until the egg whites begin to thicken. Slowly add the sugar and continue beating until stiff peaks form.
Preheat the oven to 400 degrees F. Pour the warm lemon curd into individual ramekins. Spoon the meringue onto the lemon curd being sure to fully cover the surface and out to the edges of the dish. Make some pretty swirls and peaks.
Place the ramekins on a baking sheet and bake on the middle rack of the oven for 8-12 minutes, until the meringue is browned. Remove from the oven, cool and serve, or refrigerate until ready to serve.