Peel the onions and cut them into ¼" wide rings. Separate the onion rings. Place the milk in a large bowl and put half the onion rings in the milk. Note: the milk will make the onion rings moist so the batter sticks to the onions better.

In a third large bowl, add the frying batter mix, soda water, salt, black pepper and cayenne pepper. Whisk the batter until it is smooth. Note: the soda water acts like baking soda, making the batter light and crisp when deep-fried.

Working in batches, drain the onion rings from the milk and then coat them in the flour. Shake off any excess flour, and then place the onion rings in the batter mix.

Place the onion rings in the hot oil and deep-fry until golden, about 5 minutes. Turn the onion rings during frying.

Place the onion rings on a rack placed on top of paper towels. Note: I prefer to let deep-fried foods sit on a rack rather than directly on paper towels, which I find makes the bottom of fried foods soggy.

Serve the onion rings immediately with fresh lemon wedges and homemade mayonnaise.