Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.

In a saucepan, combine water, milk, butter, and salt. Heat until it reaches a simmer.

Add flour all at once and stir vigorously until the dough pulls away from the sides and forms a ball.

Remove from heat and let cool for 5 minutes. Then beat in eggs one at a time until the dough is smooth and glossy.

Pipe or spoon 2-inch mounds of dough onto the baking sheet, spacing them apart.

Bake for 25–30 minutes until golden brown and puffed. Cool completely before filling.

In a small pot, simmer rhubarb, lemon juice, and sugar until soft (about 10 minutes). Blend until smooth.

Return to low heat and stir in egg yolk. Cook gently until slightly thickened. Cool, then refrigerate.

Whip heavy cream with powdered sugar until stiff peaks form.

Fold half the whipped cream into the chilled rhubarb curd to create the mousse.

Slice puffs in half and fill with rhubarb mousse and a dollop of remaining whipped cream.

Dust with powdered sugar and serve.