MAKE THE WHITE CHOCOLATE AND GINGER PARFAIT (START THE DAY BEFORE):
1. Melt the white chocolate in a heatproof bowl set over a pan of barely simmering water (bain-marie), stirring until smooth. Remove from heat and allow to cool to room temperature — it should be liquid but not hot. If the chocolate is too warm when folded into the cream, it will melt the mousse.
2. In a large bowl, whip the cold heavy cream to soft peaks — it should just hold its shape but still be billowy. Do not over-whip. Refrigerate while you make the sabayon.
3. In a small saucepan, combine the sugar and water. Heat over medium, stirring until the sugar dissolves, then stop stirring and bring to 118°C / 244°F (soft ball stage) on a candy thermometer.
4. Meanwhile, whisk the egg yolks in a stand mixer (or with a hand mixer) on medium-high speed until pale and slightly thickened, about 2 minutes.
5. With the mixer running on medium speed, pour the hot sugar syrup in a slow, steady stream down the inside of the bowl (avoid hitting the whisk directly, which can splatter). Once all the syrup is incorporated, increase speed to high and whisk for 5–6 minutes until the mixture is very thick, pale, mousse-like, and cool to the touch. This is the pâte à bombe base.
6. Gently fold the cooled melted white chocolate into the pâte à bombe until evenly combined.
7. Fold in the whipped cream in three additions — the first addition loosens the mixture, the second and third are folded gently to preserve volume. Fold in the diced stem ginger, ginger syrup, vanilla, and salt.
8. Pour into a loaf tin, individual dariole molds, or silicone molds lined with cling film. Smooth the top and freeze for a minimum of 8 hours, ideally overnight. The parfait must be completely frozen solid before assembly.

POACH THE RHUBARB:
9. Cut the rhubarb into batons about 7cm (3 inches) long and 1cm (½ inch) wide. Consistency of size matters for even cooking and elegant plating.
10. In a wide, shallow saucepan, combine the sugar, water, lemon juice, ginger slices, and vanilla. Bring to a gentle simmer, stirring until the sugar dissolves.
11. Lay the rhubarb batons in the simmering syrup in a single layer. Poach very gently — the liquid should barely tremble, never boil — for 4–8 minutes depending on the thickness and freshness of the rhubarb. The batons should be tender but still hold their shape with a slight bite.
12. Using a slotted spoon, carefully transfer the rhubarb to a plate. Reserve the poaching syrup — return it to the heat and reduce by half to a light, pink-tinged syrup. Strain out the ginger and vanilla pod. Cool both the rhubarb and syrup, then refrigerate.

MAKE THE ROSEMARY SABLÉ BISCUITS:
13. Combine the flour, powdered sugar, and salt in a bowl. Add the cold butter cubes and rub in with fingertips until the mixture resembles fine breadcrumbs.
14. Add the very finely minced rosemary and lemon zest. Toss to distribute evenly. The rosemary must be minced almost to a paste — large pieces create an unpleasant woody texture. A generous amount of fresh rosemary plus lemon zest amplifies herb perception.
15. Add the egg yolk and mix until the dough just comes together. Do not overwork.
16. Shape into a flat disc, wrap in cling film, and refrigerate for at least 30 minutes.
17. Preheat oven to 165°C / 325°F (fan 150°C / 300°F).
18. Roll the dough out on a lightly floured surface to approximately 5mm (¼-inch) thickness. Cut into rounds using a 7cm (3-inch) cutter. Place on a parchment-lined baking sheet.
19. Bake for 14–18 minutes until the sablés are pale gold at the edges but still slightly soft in the center — they firm up significantly as they cool. The low baking temperature preserves the rosemary's aromatic oils; high heat would burn them off.
20. Cool completely on the tray. The finished sablés should be sandy, crumbly, and fragrant with rosemary.

MAKE THE ITALIAN MERINGUE:
21. Combine the sugar and water in a small, clean saucepan. Heat over medium, stirring until the sugar dissolves, then stop stirring and attach a candy thermometer. Bring to 118°C / 244°F (soft ball stage).
22. When the syrup reaches about 110°C / 230°F, begin whisking the egg whites and cream of tartar in a stand mixer on medium speed until soft peaks form. You want the whites at soft peaks right when the syrup hits temperature.
23. With the mixer running on medium speed, pour the hot syrup in a thin, steady stream down the side of the bowl. Once all the syrup is added, increase to high speed and whip for 8–10 minutes until the meringue is thick, glossy, and cooled to just warm to the touch. It should hold stiff, shiny peaks.
24. Transfer immediately to a piping bag fitted with a large open star tip (such as a Wilton 1M or 6B). Use as soon as possible — Italian meringue holds for about 30 minutes at room temperature before it begins to deflate.

ASSEMBLE AND SERVE:
25. Remove the parfait from the freezer. If using a loaf tin, unmold and cut into neat portions. If using individual molds, pop them out. The parfait must be frozen completely solid.
26. Work quickly from this point — the parfait must stay frozen throughout assembly.
27. On each chilled plate, place a rosemary sablé biscuit slightly off-center. Set a slice or cylinder of frozen parfait on top of the sablé.
28. Pipe Italian meringue over and around the parfait, covering it completely. The meringue insulates the parfait from the torch — this is the science of baked Alaska. Aim for an elegant coating that covers the parfait rather than a mountain of meringue.
29. Using a kitchen blowtorch, toast the meringue until golden brown with charred tips in places. The toasty, nutty aroma comes from the Maillard reaction on the meringue's surface — do not be afraid of some color.
30. Arrange 2–3 batons of cold poached rhubarb alongside or leaning against the base.
31. Drizzle the reduced rhubarb poaching syrup around the plate for color and a hit of sharp acidity.
32. Garnish with a small sprig of fresh rosemary and a few dice of candied ginger, if using.
33. Serve immediately — the clock is ticking on the frozen parfait.