Preheat oven to Gas 5, 190ºC, fan 170ºC. Oil the roasting pan and put in the oven to heat for 3 minutes. Place pork in the hot oil and roll to lightly seal. Add rosemary and return to oven for 20 minutes.
Remove and turn the pork over then return to the oven for a further 10 minutes.
Cut the rhubarb into 4cm (1½in) lengths, add to the roasting pan with 100ml (3½fl oz) water and cook for a further 10 minutes. Remove from the oven and leave to rest for 5 minutes before serving.
To serve, carve the pork and spoon over the soft rhubarb and juices.
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