Cheesy Carrot Casserole

8 cups sliced carrots (about 2 pounds)
1 small onion, chopped
1 tablespoon margarine or butter
1 (10 3/4 ounce) can cream of mushroom soup
3/4 cup grated Parmesan cheese
1/2 cup breadcrumbs

Cook carrots in water until tender. Drain.

Meanwhile, in a small skillet, saut