1. In a small bowl, sprinkle gelatin over cold water; let stand until softened, 5 minutes. Add boiling water and 1 cup sugar; stir until sugar has dissolved and mixture is clear. Stir in lemon zest and juice. Set mixture over a bowl of ice and let stand, stirring occasionally, until it becomes syrupy, about 20 minutes. (If gelatin sets too much, microwave it in 10-second increments and whisk until smooth.)

2. In a large bowl, beat egg whites on medium-high to soft peaks, 3 to 4 minutes. Add gelatin mixture and continue beating until tripled in volume and thick enough to hold a ribbon on top of mixture when beater is lifted from bowl, 7 to 9 minutes. Transfer pudding to a large serving bowl and refrigerate until set, at least 4 hours and up to 2 days.

3. Combine milk, 1/2 cup sugar, tarragon sprigs, vanilla bean and seeds, and salt in a saucepan. Heat over medium, stirring occasionally, until scalding and small bubbles form at edges of pan, about 12 minutes (do not let boil). Remove from heat, cover, and let steep 10 minutes.

4. In a heatproof bowl, whisk yolks with remaining 1/2 cup sugar to combine. Remove tarragon and vanilla bean from milk mixture, then slowly whisk milk mixture into yolk mixture. Return to saucepan and cook over medium, stirring, until mixture thickens slightly and a thermometer reads 170 degrees, 5 to 7 minutes. Strain creme anglaise through a fine-mesh sieve into a bowl and refrigerate until cold, at least 1 hour or, stored in an airtight container, up to 2 days.

5. Pour 3 to 4 tablespoons creme anglaise into bottom of each serving dish. Top with a large spoonful of pudding; sprinkle with a few tarragon leaves and serve.