Prepare onion rings by taking the skin off the onion and carefully cutting into even rings. Pour buttermilk into a large bowl and place onion in the buttermilk. Leave to coat for an hour.
Pour flour and all of the spices into a large bowl. After an hour, take the onion rings out of the buttermilk, drain and dredge in the flour mixture to coat.
In a large heavy based saucepan, heat vegetable oil to 175 degrees (best to use a thermometer to reach and maintain this temperature).
Cook onion rings in small batches until golden brown and crispy.
To make the ranch sauce, add all of the ingredients in a blender and blitz until combined and smooth.
Place sauce in a small container on a plate with onion rings. Serve immediately.
