1) Preheat the oven to 190°C/gas mark 5.
2) Peel the pears and slice them lengthwise into halves. With a small, sharp paring knife and a melon ball scoop, remove the core and seeds from each pear, leaving a round well for the filling. Trim a small slice away from the rounded sides of each pear half so that they will sit in the baking tray without wobbling. Toss the pears with some lemon juice to prevent them from turning brown. Arrange them, core side up, in a baking tray large enough to hold the pears snugly.
3) Gently toss the crumbled blue cheese, dried cranberries and walnuts together in a small bowl. Divide the mixture evenly among the pears, mounding it on top of the indentation.
4) In the same small bowl, combine the apple cider, port and brown sugar, stirring to dissolve the sugar. Pour the mixture over and around the pears.
5) Bake the pears, basting occasionally with the cider mixture, for 30 minutes or until tender. Set aside until warm or at room temperature.
6) Just before serving, whisk together the olive oil, lemon juice and 3 tablespoons of the basting liquid in a large bowl.
Divide the rocket among six plates and top each with a pear half. Drizzle each pear with some of the dressing, sprinkle with salt and serve warm.