Bosc pears are strong-flavored and hold their shape when cooked, making them well suited for this savory side dish. It's a terrific accompaniment for a glazed ham or grilled sausage or most any roasted meat or poultry.
Fill a large bowl three-quarters full with water; add a handful of ice cubes. Cut onion into 16 wedges, place in a strainer and lower into the water. Let stand for 20 minutes.
Preheat oven to 400°F.
Halve and core each pear; cut each half into 6 slices. Drain the onion wedges well and place them in a 9-by-13-inch glass baking dish along with the pear slices, 1 tablespoon oil, thyme, salt and a grinding of pepper; toss to combine. Cover with foil.
Roast for 30 minutes, stirring twice.
Meanwhile, combine breadcrumbs and cheese in a small bowl. Drizzle with the remaining 2 tablespoons oil; stir to combine. Remove the pan from the oven, sprinkle the crumb mixture evenly over the gratin, return to the oven and roast until the breadcrumbs are well browned, 20 to 30 minutes more. Let cool for 10 minutes before serving.
Note: We like to use the Ian's brand of coarse dry breadcrumbs, labeled "Panko breadcrumbs." Find them in the natural-foods section of large supermarkets. To make your own breadcrumbs: Trim crusts from firm sandwich bread. Tear the bread into pieces and process in a food processor until coarse crumbs form. One slice makes about 1/3 cup. Spread the breadcrumbs onto a baking sheet and bake in a 250°F oven until dry and crispy, about 15 minutes.