Place a deep 23cm loose-based fluted tart tin onto a baking tray.
Pulse flour and butter in a food processor until fine breadcrumbs form, then blend in the beaten egg to form a ball.
Roll pastry on a floured surface into a circle 5cm larger than the tin and about 3mm thick.
Line the tin with pastry, trim the edges, prick the base with a fork, and chill for 30 minutes.
Preheat the oven to 200°C and line the pastry with parchment and baking beans.
Bake blind for 25 minutes, remove beans and paper, then bake for 3 more minutes until dry.
Reduce oven temperature to 160°C and patch any holes with reserved pastry.
Whisk eggs, egg yolks, and sugar in a heatproof bowl until smooth.
Stir milk, cream, vanilla pod, and seeds in a saucepan.
Heat liquid until hot, then slowly whisk into the egg mixture and pour into the pastry case.
Bake for 40-45 minutes until the custard is only just set and still slightly wobbly.
Cool the tart completely and chill in the fridge for at least 2 hours.
Dissolve sugar in 15ml water in a pan and cook rhubarb in a single layer for 4 minutes per side.
Transfer rhubarb to a bowl and chill until cold.
Remove the tart from the tin, top with rhubarb, and serve.