Another reason to not soak the onions first is that batter sticks better to a dry onion. Some cooks will even slice them and leave them in the refrigerator overnight to dry, but we haven't found this necessary when frying battered onion rings. What you really want is a thick batter. At Melvin's, their batter is so thick that the onion slices will sit on top of the batter without sinking in, as seen in the screenshot below. The image above is taken from a video posted by Melvin's. See how the sliced onion sits on top of the batter. It will not sink. The cook must push the onion below the batter. Because of the thickness, the batter will not slip off.