For the pastry, sift the flour into a large bowl and add the cold butter. Using your fingertips gently rub the butter into the flour until it resembles fine breadcrumbs.
Stir in the almonds, rosemary and sugar. (You could use a food processor and pulse a few times instead of doing it by hand.) Add the yolk and bring together with your fingers but don’t overwork.
Wrap in cling film and place in the fridge for 30 minutes.
Preheat the oven to 170C/325F/Gas 3. Place a 23cm/9in fluted flan tin on a baking tray.
For the rhubarb sauce, put the rhubarb, sugar, cinnamon and four tablespoons of water in a saucepan and bring to the boil. Simmer until the rhubarb is soft, then remove from the heat and allow to cool a little.
Discard the cinnamon and, using a hand held blender or food processor, blend the rhubarb to a purée. Pass through a sieve and pour into a squeezy sauce bottle or piping bag and set aside to cool.
On a lightly floured work surface, roll out the pastry thinly. (It will be very hard to work with but you can patch it up in the tin if it tears.) Line the tin with the pastry, making sure the edges aren’t too thick. Alternatively, shape the pastry into a disc, place in the tin and the use your fingers to press the pastry into the shape of the tin. Transfer to the fridge to chill for a few minutes.
Meanwhile, cut a piece of baking parchment large enough to line the pastry. Scrunch it up into a ball then open it out. This softens it, ready for blind baking the pastry case.
Remove the pastry case from the fridge and prick all over with a fork. Put the softened baking parchment inside and fill with baking beans or uncooked rice. Make sure the edges of the pastry case are covered with the baking parchment and it is overhanging - this will protect the pastry from burning. Bake for 15 minutes, or until the pastry is dry and pale golden-brown.
Remove the baking parchment and beans. Brush the egg wash over the pastry and return to the oven for another 5 minutes. Remove from the oven and allow to cool.
Turn the oven down to 160C/325F/Gas 3.
For the custard, whisk the eggs and sugar together in a bowl. In a saucepan, bring the cream to the boil then add it to the egg and sugar mixture. Whisk until well combined and the sugar is dissolved. Pass through a fine sieve into a jug.
Once the pastry case is cool, pipe a thick swirl of rhubarb purée into the pastry case. Pour half of the custard over the rhubarb before transferring the pastry case to the middle shelf of the oven. Carefully pour in the remaining custard while the tart is in the oven (this will prevent the filling spilling over the pastry when you lift it into the oven).
Bake for 10 minutes, then carefully remove the tart (the filling will still be very wobbly) and squeeze the remaining rhubarb purée in a big swirl over the custard. Return the tart to the oven and bake for a further 25 minutes, or until the custard is set but still has a gently wobble in the centre. Leave to cool in the tin before turning out.
Grate the nutmeg over the top and serve.