At Iowa's Rapid Creek Cidery, chef Matt Steigerwald serves this gratin (which tastes sort of like the best French onion soup ever) with grilled artisan bread. Crisp and charred, it's a good foil to the gooey richness.
Preheat oven to 425°. Place onions, apples, oil, thyme, salt, crushed red pepper, and black pepper in a 15x10x1-inch baking pan. Toss to combine. Roast until browned and very tender, 20 to 25 minutes.
Meanwhile, generously butter a 1 1/2 quart baking dish. (Or butter six 6- to 8-ounce individual casserole dishes.) Set aside.
Add bread, 1/4 cup cheese and the sage to onion mixture. Stir to combine. Transfer mixture to prepared baking dish; pour cream over mixture. Top with remaining 1/4 cup cheese. Bake until bubbly and brown, 25 to 30 minutes. Let stand 10 minutes before serving.