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Heat the frying oil to about 180 degrees Celsius or 350 degrees Fahrenheit, I prefer to use a heavy bottomed pan. If you don't have a thermometer to check the exact temperature you can also check with a small piece of paper kitchen towel.
When you start to see bubbles, it means the oil is hot enough. Meanwhile peel the onions, divide them into thick rings and loosen them up. Place some paper kitchen towels in a bowl.
A simple truffle mayonnaise can be made by combining 3 tablespoons mayonnaise with 3 tablespoons plain yogurt and season with a bit of lime juice, pepper, salt and a teaspoon truffle paste.
Combine the egg along with beer and salt and then gradually add the flour, stir with a whisk to create a smooth batter. For extra flavouring you can also add some garlic powder.
Coat the onion rings one by one with the batter and carefully add them to the oil. Fry them until beautiful golden brown on both sides. Don't overcrowd the pan otherwise they will stick together.
Transfer them to the bowl with paper kitchen towels to remove any excess oil and serve immediately, e.g. with garlic sauce or truffle mayonnaise. Enjoy!