1) Preheat oil in a deep-fryer or Dutch oven to 180 degrees C.
2) To make the chilli sauce, combine the mayonnaise, chilli sauce, chilli powder and cayenne in a small bowl and stir well. Cover and chill until serving time.
3) Slice the onions into very thin rings and separate the rings. Spread them out and sprinkle with seasoned salt. Place the flour in a resealable plastic bag. Put the onion rings in the bag in batches, close tightly, and shake until the onion rings are coated.
4) Fry the onion rings in batches for 3 to 4 minutes, until they are brown and crispy. Remove with tongs and drain on paper towels.
5) Taste and season with salt, if necessary. Serve with chilli sauce.