Add the ground almonds, grated parmesan, salt, baking soda and lemon juice to a food processor. Pulse the ingredients together briefly to combine. Crack the egg into the food processor and add the olive oil. Blend to combine. You should have a thick, smooth batter. Transfer the batter to a bowl. If your batter feels too thick you can add a little more oil.
Peel the onion. Very carefully, slice the onion into half inch thick slices, working across the onion to create rings. Separate the rings with your fingers, reserving any smaller inner pieces for another meal. You want to have 16 onion rings.
Arrange the onion rings across a clean work surface. Dredge each onion ring in the batter so they are all coated in a thin layer.
Lightly oil your air fryer basket. Arrange the coated onion rings in the base of your air fryer basket. You will need to cook in batches, but should be able to fit in roughly half the onion rings at a time.
Place the basket in the air fryer. Set the temperature to 375 degrees Fahrenheit. Set the timer for 8 minutes. Remove the basket from the air fryer and carefully turn the onion rings over. Set the timer for a further 4 minutes. The onion rings should be golden all over and crisp at the edges. Return to the air fryer to cook further if needed. Repeat this process with the remaining onion rings.