Heat the oven to 180C/160C fan/gas 4. Line a roasting tin and arrange the rhubarb in a single layer. Scatter over the sugar, cardamom, blood orange zest and juice, and the lemon juice. Cover and roast for 20 mins. Remove from the oven, uncover and leave to cool in its syrup.

Scoop out and discard the cardamom pods, then use a slotted spoon to transfer half the rhubarb into the bowl of a food processor. Add a couple of spoonfuls of the syrup and blitz to a purée.

Combine the purée with the condensed milk and a pinch of salt. Stir in the ginger. Whip the cream to medium peaks in a bowl, then carefully fold in the rhubarb mix.

Spoon the mousse into glasses and chill for a few hrs or overnight until set. Top with the remaining rhubarb, along with its syrup and scatter over the pistachios.