Preheat oven to 375 degrees F. Toss pears with the other ingredients, except mint and ice cream. Place in a single layer, core-side up in a baking dish that's just large enough to hold them. Cover and roast until pears are just tender, about 25 to 30 minutes. Remove from the oven and let them cool slightly. Arrange one pear half on each of the 4 dessert plates, and then brush or drizzle - thin with a drop or two of water if vinegar is very reduce - with the vinegar in the pan. Place a scoop of ice cream in the middle of each pear, garnish with mint, and serve.