Place the pears in a pan large enough to accommodate them comfortably. Add the lemon juice, sugar, sprig of rosemary and enough water to cover the pears.
Bring to the boil, turn down the heat and simmer gently until the pears are soft.
While the pears are cooking, make the caramel sauce. Tip the sugar into a non-stick pan and heat gently until the sugar is dissolved and caramelised. Do not be tempted to start stirring the sugar until this stage is reached. Add the butter, rosemary and cream and stir well. (Take great care at this stage because the molten sugar is extremely hot and can cause serious burns if it splashes onto your skin).
Remove the sauce from the heat and leave to cool. Remove the rosemary sprig before serving.
To serve, slice the chilled pears in half lengthways, place in the centre of the plate. Add a scoop of good vanilla ice cream and a drizzle of the caramel sauce.