To Make the Shortcrust Pastry:
1. Start by whisking together the flour, powdered sugar, and salt in a big bowl until they're nicely combined.
2. Now, add the cold butter cubes. Gently rub them into the flour mixture with your fingertips until it resembles coarse breadcrumbs, with a few pea-sized butter pieces remaining. This keeps the crust flaky!
3. Pour in the egg and cold water. Mix lightly with a fork until the dough just starts to come together.
4. Use your hands to gently bring the dough into a ball. If it’s too crumbly, add a tiny bit more cold water, a few drops at a time, until it forms a cohesive ball. Avoid overworking!
5. Lightly flour your workspace. Shape the dough into a flat disc, wrap it in plastic, and chill in the fridge for at least one hour. This makes it easier to roll and helps the crust stay crisp.
6. Take the dough out of the fridge and let it sit at room temperature for 15 minutes. Preheat your oven to 350°F (175°C).
7. On a lightly floured surface, use a rolling pin to roll the dough into a 12-inch circle, about ⅛ inch thick.
8. Carefully fit the dough into a 9-inch tart pan with a removable base. Press it gently into the bottom and sides, then trim any excess with a sharp knife. Prick the bottom all over with a fork to prevent puffing.
9. Line the crust with parchment paper and fill it completely with pie weights or dry rice.
10. Bake for 15 minutes. Then, remove the parchment and weights and bake for another 15 minutes, or until the crust is a light golden brown and feels dry. Let it cool completely.
To Make the Lemon Filling:
1. In a small bowl, whisk together the fresh lemon zest, lemon juice, and cornstarch until smooth and lump-free.
2. Bring the water to a boil in a medium saucepan. Stir in the lemon-cornstarch mixture, then reduce heat to medium-low and stir constantly until it thickens. Remove from heat and set aside.
3. In a separate bowl, whisk the granulated sugar and egg yolks until smooth. Gradually whisk this into the lemon mixture. Return the pan to low heat and gently bring to a low boil, stirring often, until the filling becomes very thick (about 5 minutes, or longer).
4. Remove from heat, let cool for 10 minutes, then pour into the cooled tart shell. Chill in the refrigerator for at least 1 hour, or until firm.
To Make the Meringue:
1. In a clean stand mixer bowl with the whisk attachment (or using a hand mixer), beat the egg whites on low speed until frothy, then increase to medium and beat until soft peaks form.
2. Gradually add the superfine sugar, one tablespoon at a time, ensuring each addition is fully incorporated before adding more. Once all sugar is in, scrape the bowl, then beat on high for 5 minutes until the meringue is glossy, thick, and holds stiff peaks (no sugar grains should be felt when rubbed between fingers).
3. Once the lemon filling is set, spoon or pipe the beautiful meringue over the top, creating decorative swirls. Use a kitchen torch to lightly brown the peaks, or briefly place it under a hot broiler (watch carefully!).
4. For a lovely finish, garnish with fresh lemon slices and a few sprigs of lemon thyme, if desired.
5. Slice, serve, and enjoy this delightful, bright tart!