Place a deep 23cm/9in loose-based fluted tart tin onto a baking tray. (It needs to be around 4mm deep.)
To make the pastry, put the flour and butter into a food processor and pulse until the mixture resembles fine breadcrumbs. With the motor running, gradually add the beaten egg and blend until the mixture forms a rough ball.
Tip the pastry onto a floured surface and roll out into a circle roughly 5cm/2in larger than the tart tin and around the thickness of a £1 coin. Turn the pastry 90 degrees every couple of rolls to create an even thickness, rubbing the rolling pin with flour every now and then so it doesn’t stick. Lift the pastry over the rolling pin and sprinkle a little flour underneath as you roll.
Lift the pastry over the rolling pin and lower gently into the flan tin. Push the pastry into the base and sides of the tin. Trim the pastry by pinching between your thumb and finger, lifting slightly above the rim of the tin. Roll the trimmings into a ball and wrap in cling film to use for patching the cooked tart if necessary. Prick the base lightly with a fork and chill in the fridge for 30 minutes.
Preheat the oven to 200C/180 (fan)/Gas 6. Place the chilled pastry case onto a clean baking tray. Line the inside with crumpled baking parchment and half-fill with baking beans or dried beans.
Bake blind for 25 minutes, then carefully take out of the oven and remove the paper and beans. Return to the oven for a further three minutes, or until the surface of the pastry is dry.
Remove the pastry case from the oven and reduce the temperature to 160C/140C (fan)/Gas 3. If the pastry has any holes, simply patch with the reserved pastry to stop any of the filling leaking out.
For the filling, whisk the eggs, egg yolks and sugar in a heatproof bowl with a metal whisk until smooth. Set aside
Put the milk and cream into a medium saucepan. Stir the split vanilla pod and seeds into the milk and cream.
Heat the liquid gently until hot but not boiling, stirring regularly. Slowly stir it into the egg mixture until thoroughly combined. Pour the custard into the prepared pastry case.
Bake in the oven for 40-45 minutes, or until only just set. The custard should still be fairly wobbly in the middle, as it will continue to set as it cools.
Remove the tin from the oven and leave the tart to cool completely. Chill in the fridge for at least two hours.
For the rhubarb topping, place the sugar and one tablespoon of water in a large non-stick frying pan over a low heat. Stir until the sugar is dissolved. Add the rhubarb to the pan in a single layer. Cook gently for four minutes, then turn over with a spatula and cook on the other side for a further 3-4 minutes, or until softened but not falling apart.
Remove the rhubarb from the pan and tip into in a large heatproof bowl. Add a little extra sugar, to taste, if needed, and leave to cool. Chill in the fridge until needed.
Just before serving, slowly release the tart from the tin. Transfer to a serving plate or board. Spoon the rhubarb on top of the tart and serve immediately, with extra cream for pouring if you like.