An elegant, airy mousse built on a fresh-ginger curd lightened with whipped cream and set with a touch of gelatin. Intensely gingery but silky and cloud-light.
Cover the ginger with the 1/2 cup water in a small saucepan; cover and bring to a boil. Reduce the heat and simmer 2 minutes, covered, then let stand about an hour to infuse and cool. Strain the ginger water through a fine sieve, pressing hard on the solids with a spoon to extract as much as possible.
Sprinkle the gelatin over the remaining 4 teaspoons of water in a microwave-safe dish. Let it soften while you do the next step.
Whisk the sugar and egg yolks together in a heatproof bowl. Gradually whisk in the ginger "juice." Set the bowl over a saucepan of simmering water and cook, whisking constantly, until the mixture thickens, about 10 minutes.
Microwave the gelatin on high 30–40 seconds until dissolved, and transfer to the bowl of a stand mixer. Add the ginger curd (strain it through a fine sieve if it has lumps) and beat with the whisk attachment until thick and cooled to room temperature, about 5 minutes, scraping down the bowl once or twice. Scrape into a large bowl.
Add the cream to the stand mixer and whip to stiff peaks (no need to clean the bowl first). Gently fold 1/3 of the whipped cream into the curd to lighten it, then fold in the remaining cream. Spoon the mousse equally into 4 (or 6 smaller) serving dishes and refrigerate 1 hour, until chilled and set, or up to 24 hours ahead.
Notes
Infuse, don't rush: Let the ginger steep about an hour off the heat for deep, mellow flavour, then press the solids hard to extract every drop.Bloom the gelatin: Soften it in cold water first, then melt — that prevents grainy bits in the set mousse.Cook the curd gently: Over simmering (not boiling) water, whisking constantly, so the yolks thicken without scrambling.Lighten, then fold: Stir in a third of the whipped cream to loosen the curd, then fold in the rest gently to keep it airy.