Heat the oven to 160C fan/gas mark 4. Line a medium-sized baking tray with baking parchment. To make the sponge, cream the butter and sugar together in a mixing bowl until pale. Beat in the eggs, then sift over the flour and sprinkle in the poppy seeds.
Fold the sponge mixture together carefully, then spoon it onto the prepared baking sheet and level it off – it shouldn’t be deeper than 2-3cm. Place in the oven to bake for about 15-20 minutes or until the sponge is golden and cooked through.
To cook the rhubarb, slice the stems into 3-4cm pieces and tumble them with the water, lemon zest and juice, and the sugar, and turn out onto a baking tray.
Arrange the rhubarb in a single layer if you can, and pop into the oven to bake for 25 minutes or until the stems are tender but are still holding their shape. Set the rhubarb aside to cool.
To make the lemon custard, pour the milk into a heavy based saucepan with the lemon zest and set over a medium heat. Bring this mixture to simmer.
Combine the sugar, cornflour and the egg yolks in a bowl and whisk until well combined. Pour over the hot milk, whisking as you do.
Return this mixture to a clean pan. Gently cook the custard until it starts to thicken up. Make sure you keep stirring to avoid overcooking it. Take off the heat and stir in the lemon juice.
Pass the custard through a sieve then allow it to cool. Whip the cold cream until it forms soft peaks then fold or whisk the cream into the cooled custard.
Assembling the trifle is easy. Slice the sponge cake up into squares or fingers and arrange these over the base of large serving bowl, preferably a glass one.
Arrange the cooled roast rhubarb over the sponge and spoon over all the pink roasting juices. Pour the chilled lemon custard over the rhubarb then place the trifle back in the fridge to chill for an hour or so.
To finish, whip the cream and the 25g of sugar to smooth soft peaks with a whisk. Don’t overwork it or it will go grainy. Spoon the soft cream over the custard. Return the trifle to the fridge until you’re ready to serve it.
Sprinkle over the toasted almonds and a few more poppy seeds before bringing to the table.