In a large bowl, combine the cubed pork, beef, and lamb. Add minced garlic, thyme, bay leaves, and season generously with salt and pepper. Pour in the white wine, cover, and marinate in the refrigerator for at least 6 hours or overnight.
Preheat your oven to 325°F (160°C).
Grease a large, ovenproof casserole dish with butter. Layer half of the sliced onions at the bottom of the dish.
Arrange half of the marinated meat on top of the onions, followed by half of the sliced potatoes, carrots, and leeks.
Repeat the layering with the remaining ingredients, finishing with a layer of potatoes on top.
Pour the chicken or beef broth over the layered ingredients.
Cover the casserole dish with a lid or aluminum foil and place it in the preheated oven.
Bake for 2 to 3 hours, or until the meat is tender and the flavors have melded together.
Serve hot, garnished with fresh parsley. Baeckeoffe Alsace is traditionally enjoyed with a crisp Alsatian white wine.