Generously butter a 2-quart baking dish (a 9×5-inch loaf, 8- or 9-inch square dish, etc.) with 1 tablespoon butter. Melt remaining 4 tablespoons butter in large skillet over medium heat. Add onion, thyme, salt and lots of freshly ground black pepper and cook for 2 minutes, until becoming translucent. Add celery and cook for 2 more minutes. Add apple and saute until a bit tender, 3 to 4 minutes more.