Giant rhubarb and pomegranate millefeuille

Method

1. First, poach the rhubarb. Put it in a wide stainless steel (or other non-reactive material) saucepan. Stir in the sugar and leave to macerate for 20 minutes.
2. Cover the macerated rhubarb with the grenadine mixture and put over a low medium heat until it reaches a very gentle simmer (don’t boil as the rhubarb will lose its shape). Cook for 7 minutes, then remove from the heat, cover with a lid and leave to steam for 5 minutes until almost cooked through. Remove the lid and leave to cool for 15 minutes, then carefully transfer to a container. Leave to cool completely, then cover and chill for 12-24 hours. The colour will intensify over time.
3. Make the custard for the pastry cream, too, so you can also chill it overnight. Put the milk and vanilla in a small pan and heat gently. Put the egg yolks and sugar in a medium bowl and whisk until combined and slightly lighter in colour, then whisk in the cornflour. As soon as the milk starts to steam and quiver (but isn’t quite boiling), remove from the heat and gradually pour it over the egg mixture while whisking constantly. Once it’s all combined, tip the mixture back into the pan and put over a medium heat. Cook for about 10 minutes, stirring constantly but slowly with a rubber spatula until very thick. Scrape into a bowl and cover the surface directly with a piece of baking paper. Set aside to cool, then chill overnight.
4. Heat the oven to 220°C/200°C fan/gas 7. Unroll the pastry sheets and transfer each to a baking tray (still on its baking paper), then sit another baking tray on top of two of them. This will stop them puffing up too much as they cook – but we want the third sheet, which will be the top layer, to puff. (If you don’t have enough trays, do this in batches.) Bake for 25-30 minutes until a deep golden brown, dusting the puffed-up top pastry sheet with icing sugar for the final 10 minutes of cooking. You want the pastry to be dark and well caramelised rather than pale and light, and the sugar on the puffed sheet to have dissolved and caramelised, giving the pastry a shiny surface. Set aside to cool completely.
5. Use a slotted spoon to remove the rhubarb from its poaching liquid into a bowl. Transfer the liquid to a small pan and boil for 3-5 minutes until reduced to a syrup (about 50ml). Set aside to cool.
6. Remove the custard from the fridge and use a balloon whisk to mix it so it’s less stiff. Whisk the cream to medium peaks (the top of the peaks flop slightly when the beaters/whisk are lifted out) in a large bowl, then mix in the custard. Transfer the resulting pastry cream to a piping bag (you may need to do half at a time) – we fitted ours with a very large ribbon piping nozzle but you could use a large round nozzle if you prefer.
7. To assemble the millefeuille, pipe ribbons of the pastry cream across the surface of one of the non-glazed sheets of pastry. Reserve about 10 pieces of rhubarb to decorate the top, then spoon half the remaining rhubarb over the pastry cream and scatter over some pomegranate seeds. Pour on a little of the reduced syrup. Sit another pastry rectangle on top and repeat. Top with a glazed pastry rectangle. Once assembled, pipe any remaining custard cream on top with the rhubarb pieces and pomegranate seeds to decorate. You could also add some rhubarb ribbons (see tip). Sift over some icing sugar and serve immediately.