Lemon Curd
In a heat-safe bowl, whisk together all lemon curd ingredients except the butter. Place the bowl over a pot of simmering water, ensuring the bowl doesn’t touch the water.
Continuously whisk for about 10 minutes to prevent the eggs from cooking separately. The mixture should thicken slightly.
Gradually whisk in the cold butter cubes until fully melted. Attach a candy thermometer to the bowl and continue whisking for an additional 5 minutes, or until the mixture reaches 175˚ F (or a yogurt-like consistency if not using a thermometer).
Once ready, immediately pour the lemon curd into your pre-baked tart shells. Let the curd cool and set as it thickens further.
Meringue
Attach a candy thermometer to a small saucepan, ensuring the tip is not touching the bottom. Add sugar and water, then bring to a boil over high heat.
While the syrup is heating, begin whisking egg whites on low speed using an electric mixer. Ensure the bowl and beaters are clean and grease-free.
When the sugar and water reach 250˚ F, the egg whites should have a foamy consistency. Gradually add the hot syrup to the egg whites, one tablespoon at a time, while continuing to whisk on high speed.
Beat the mixture on high for 6-8 minutes until the meringue cools and forms stiff, glossy peaks resembling shaving cream.
Spread the meringue over the cooled lemon curd, ensuring it covers the tart completely. Optionally, use a kitchen torch to lightly toast the meringue for a beautiful golden finish.