For the onion rings, mix together the flour and water to make the batter consistency of single cream.
Heat the oil in a deep, heavy-based saucepan. Dip the onion rings into the batter and then drop in batches into the hot oil for 2-3 minutes. (CAUTION: Hot oil can be very dangerous. Do not leave unattended.)
Remove and drain on kitchen paper.
For the sauce, place all of the ingredients in a saucepan and bring to the boil over a medium heat.
Let simmer for 2-3 minutes. Using a hand blender, blend ingredients until smooth.
Serve with the onion rings.