Add the milk, butter and 125ml water to a medium saucepan over a medium heat. Heat until the butter has melted, then bring it to a rolling boil.
Remove from the heat and immediately add the flour and sugar, beating with a wooden spoon until the mixture is smooth, comes together and easily comes away from the side of the pan. Transfer to a large plate and spread out, leaving until cool enough to handle by hand. Once ready, transfer back to the pan and beat in the eggs, one by one, until smooth and glossy.
Preheat the oven to 220°C/gas mark 7. Mark two large pieces of baking paper with two rows of lines, 9cm in length and spaced evenly part. Turn them over and line two baking trays with the paper.
Using a nozzle with a width just over 1cm, fill a piping bag with the dough and pipe on to the trays, using the lines as guides. Transfer the trays towards the top of the oven and bake for 25 minutes until golden brown. Once baked, transfer to a wire rack and immediately, but carefully, slice down the side allowing any steam to escape while they cool, so they don’t get soggy. Set aside to cool completely.
Now start on the crème pat. Warm the milk in a large saucepan until it starts to get steamy, but doesn’t boil. Set aside. In a large bowl, whisk together the sugar and flours. Add the egg yolks and whisk them into the dry ingredients, forming a thick paste.
Pour a little of the hot milk into the egg yolk mixture and whisk to combine. Gradually add the rest of the milk slowly into the eggs, whisking continuously. When all the milk has been added to the eggs, pour everything back into the saucepan. Set a sieve over a bowl and place near the stove.
Set the pan back over a medium heat and whisk constantly until it starts to thicken. This will take a few minutes. Once it starts to bubble (you will have to pause briefly between whisking to check) remove from the heat and beat in the vanilla extract. Pour through the sieve to remove any cooked egg. Spread out on a plate, cover with clingfilm and leave to cool.
Cut the rhubarb into 5cm chunks and then slice lengthways into 4-5 pieces. In a large frying pan, add the sugar along with 225ml water and the grenadine. Stir over a medium heat until the sugar has completely dissolved, then bring to the boil. Add the rhubarb in a single layer and then turn down the heat, simmering for 2 minutes. Turn off the heat, cover, and leave for 5 minutes or until just tender.
Using a slotted spoon, place the rhubarb on a tray and transfer to a fridge to cool. Keep the leftover liquid and leave that to cool too. Once cool, puree 225g of the poached rhubarb until mostly smooth. Add to the crème patisserie along with the cinnamon and fold the two together until combined.
Make the icing by combining the icing sugar with 3 tbsp of the leftover poaching liquid, adding 1 tsp or more if it seems too stiff. Add a drop of pink food colouring if you want a brighter hue. Fill the éclair shells with the rhubarb crème pat using an icing bag and 1cm nozzle. Use a palette knife to spread the icing on to each of the eclairs. Top with the remaining poached rhubarb and crystalised ginger.