Green Bean-Cheddar Casserole

Heat covered 7- to 8-quart saucepot of salted water to boiling on high. Preheat oven to 375 degrees F. Add green beans to boiling water. Cook 2 minutes. Drain well; set aside. In food processor, pulse bread into coarse crumbs. Transfer to medium bowl along with oil and green onions. Toss to combine; set aside. In 4-quart saucepan, whisk cornstarch and 1/2 cup milk until smooth. Add nutmeg and 1/2 teaspoons each salt and pepper. Slowly whisk in remaining 2 cups milk. Heat to boiling on medium high, whisking frequently. Boil 2 minutes, whisking. Reduce heat to medium low. Stir in cheeses one handful at a time, waiting until cheese melts before adding next handful. Stir in green beans until well coated. Transfer mixture to 3-quart baking dish. Top with reserved crumb mixture. Bake 25 to 30 minutes or until crumbs are golden brown.