Turnip Casserole

1 1/2 pounds turnips, peeled and thinly sliced
2 tablespoons butter
1 onion, thinly sliced
2/3 cup chopped green bell pepper
1/3 cup chopped celery
2 tablespoons flour
1 cup milk
1/2 cup grated sharp cheese
Salt and pepper, to taste
3 tablespoons bread crumbs

Cook turnips in boiling, salted water to cover until just tender. Drain. Saut