Tip:To adapt a slow-cooker recipe to a conventional oven, follow these guidelines: add more liquid, to accommodate for greater evaporation; bring the dish to a boil over high heat in a Dutch oven, then cover the pot and put in a 350°F oven. Plan on the dish taking roughly half the time to cook in the oven as it would in the slow cooker.
Combine the zest, ginger, cinnamon, cloves, star anise, and peppercorns on a medium piece of cheesecloth and bundle into a bag, securing with kitchen twine. Set aside.
Stir together the rest of the ingredients except for the pears in the slow cooker, then add the pears, coating them with the sauce. Nestle in the spice bag. Cover and cook on low until the pears are very tender but still retain their shape, about 3 hours.
Carefully transfer the pears to a 9 x 13 x 2-inch baking dish (where they’ll later marinate in the reduced cooking liquid). Pour the cooking liquid into a medium saucepan, discarding the cheesecloth bag. Boil over high heat until reduced to a scant 1-1⁄4 cups, 25 to 30 minutes (the sauce should slightly coat the back of a spoon). Pour the sauce over the pears in the baking dish and turn to coat. Let sit, covered, in the fridge for at least a couple of hours (marinating the pears will give them even more color and flavor). The pears can be served cold, at room temperature, or warm. When ready to serve them, transfer two pear halves to each plate and top with some poaching liquid.