Preheat oven to 350ºF. Use canola oil cooking spray or butter to lightly grease the bottom of a large casserole dish. Saute garlic and onions in one tablespoon of butter, margarine, or extra-virgin olive oil. Set aside. Layer the bottom of the dish with crushed cracker crumbs, then arrange a layer of oysters and follow with butter, garlic, and onion mixture. Sprinkle with a little salt and pepper and a dusting of garlic and celery salts. Repeat the layers, ending with crumbs and butter slices arranged on the top. In a small bowl, lightly whisk together the last six ingredients and pour over the layers. If there is not enough liquid to come just under the top of the layers, mix up a little more milk, cream, and oyster liquor, and add. Bake for 30 minutes or until bubbly around the edges and golden brown on the top. Cool five to 10 minutes before serving.