Meanwhile, carefully remove pan from slow cooker base and pour liquid through a fine sieve. Discard apple-onion pulp (or serve on the side, hot, like applesauce) and return the clarified liquid to the slow cooker and continue heating, uncovered. In a small prep bowl, mix 1 tablespoon cornstarch with 2 tablespoons cold water until smooth and lump-free. Add cornstarch and water to the slow cooker and whisk constantly to combine with cooking liquid until the mixture has thickened and sauce has formed. Pour sauce over meat on platter and serve immediately.