Heat oil over med-high heat to 375°F. If you don't have a candy thermometer, to test if oil is ready, dip a wooden chopstick in the oil – if bubbles form around it, its ready.

Put all dry ingredients in a zip lock bag and shake to combine. Then put it in a large bowl, and pour in 1 full bottle/can and mix well. If too thick, add a bit more of the second bottle/can, until mixture is smooth and is not too thick or too thin (it will be like really thick pancake batter).

Dip fish fillets or onion rings in batter, making sure both sides are coated evenly, and gently place a few pieces in hot oil. Cook until golden brown, turning once (cooking times: fish - approx. 5 minutes, onion rings - approx. 2 minutes).