To Make the Easy Shortcrust Pastry:
1. In a large mixing bowl, gently combine 1 ¾ cups all-purpose flour, ¼ cup powdered sugar, and ¼ teaspoon salt until well blended.
2. Using your fingertips, delicately rub the ½ cup cold, cubed unsalted butter into the flour mixture. Continue until it looks like coarse breadcrumbs, with a few pea-sized pieces of butter remaining. This helps create a flaky crust.
3. Add the 1 large egg and 1 ½ tablespoons cold water to the bowl. Mix with a fork until just combined, then lightly bring the dough together with your hands. If it's too crumbly, add a few extra drops of cold water, just enough for it to form a soft ball.
4. Lightly flour your surface, then gently press the dough ball into a flat disc. Wrap it tightly in plastic wrap and refrigerate for 1 hour. Chilling makes the dough much easier to roll!
5. Remove the chilled dough from the refrigerator and let it rest at room temperature for 15 minutes. Preheat your oven to 350°F (175°C).
6. Lightly flour your work surface and use a rolling pin to roll the dough into a 12-inch circle, about 3mm thick. Carefully transfer the dough into a 9-inch tart pan with a removable base. Gently press the dough onto the bottom and up the sides of the pan. Trim any excess dough with a sharp knife and prick the bottom all over with a fork to prevent it from puffing up.
7. Line the tart shell with baking parchment and fill it completely with pie weights or uncooked rice. Bake for 15 minutes. Carefully remove the weights and parchment, then bake for another 15 minutes, or until the pastry is a pale golden brown and feels dry. Let it cool completely.
To Make the Luscious Lemon Filling:
1. In a small bowl, whisk together the zest and juice from 4 fresh lemons with ⅓ cup cornstarch until perfectly smooth and lump-free.
2. In a medium saucepan, bring 1 ½ cups of water to a boil over high heat.
3. Add the lemon-cornstarch mixture to the boiling water. Reduce the heat to medium-low and stir continuously until the mixture thickens visibly. Remove from heat.
4. In a separate mixing bowl, whisk 1 cup granulated sugar and 4 egg yolks until completely smooth. Gradually add this mixture to the warm lemon base, stirring gently as you go.
5. Return the pan to low heat and slowly bring it to a gentle boil, stirring constantly. It should thicken considerably in about 5 minutes. Once very thick, remove from heat and let it cool for 10 minutes.
6. Pour the cooled lemon filling into your pre-baked tart shell. Refrigerate for at least 1 hour, or until the filling is firm and set.
To Make the Fluffy Meringue:
1. In a very clean stand mixer bowl fitted with a whisk attachment, add the 4 egg whites. Beat on a low speed until frothy bubbles form, then increase to medium speed and beat until soft peaks appear.
2. With the mixer running, add the 1 cup superfine sugar, one tablespoon at a time, allowing each addition to fully dissolve before adding more. This slow addition helps stabilize the meringue for a glossy finish.
3. After all the sugar is added, scrape down the bowl. Increase the mixer speed to high and beat for 5 minutes. The meringue should be bright white, thick, glossy, and hold stiff, firm peaks when you lift the whisk. (You shouldn't feel any sugar grains when rubbed between your fingers).
4. Transfer the meringue to piping bags fitted with your favorite nozzles. Pipe beautiful swirls over the chilled lemon filling, ensuring the meringue extends all the way to the crust edges for a perfect seal. Use a chef's blow torch to lightly scorch the meringue until it's golden brown and toasted. Alternatively, you can place the tart under a hot broiler for 1-2 minutes, watching very closely to prevent burning.
5. For a final elegant touch, garnish your tart with thin lemon slices and fresh lemon thyme sprigs. Slice, serve, and savor this refreshing, homemade treat!