This beautiful fall dessert is quick to prepare and then roasts in the oven, leaving you free to get the rest of the meal together.
In a saucepan just large enough to hold pears on their sides, combine 5 cups water, vinegar, sugar, rosemary, and pepper. Bring to a boil, stirring until sugar has dissolved. Remove from heat.
Arrange pears on their sides in a single layer in poaching liquid. Cover and simmer, turning occasionally, until tender, 25 to 30 minutes. Using a slotted spoon, carefully transfer pears to a plate. If making in advance, let pears cool; then cover and refrigerate up to 3 days.
Bring poaching liquid to a boil; cook until slightly syrupy, deep brown in color, and reduced to 3/4 cup, 35 to 40 minutes. Strain through a fine-mesh sieve (discard solids). If storing, transfer syrup to a covered container, and refrigerate up to 3 days. (Store pears and syrup separately.)
Serve pears chilled or at room temperature: Cut a thin slice from the bottom of each pear so it can stand upright. Place one on each of four serving plates along with vanilla ice cream, if desired. Drizzle with syrup.
Pears are best peeled with a vegetable peeler, and best cored with a melon baller.