Bring the cream just to the scalding point, and turn off heat. Scrape in the seeds from the vanilla bean, drop in the vanilla pod and the rosemary. Set aside.

Spread the sugar in a large, heavy saucepan. Cook over medium heat, carefully watching and stirring occasionally with a heatproof spatula until melted. When the sugar is a deep amber, stir in the pears. The caramel will seize, that's okay. Continue cooking and stirring occasionally with a heat-proof spatula for ten minutes. The sugar will have dissolved.

In the meantime, remove the vanilla pod and the rosemary from the cream. Remove the caramel from the heat and stir in half a cup of the cream, then mix in the rest of the cream, salt, and lemon juice.

Cool to room temp, purée until smooth, and press through a strainer. Chill according to your ice-cream maker manufacturer's instructions. Be sure not to over churn or you'll have buttery bits in there.