Place the flour in a large bowl. Make a well in centre and break the egg into it.
Beat lightly then gradually pour in the milk.
Continue beating milk and egg, incorporating all the flour, to make a smooth batter.
Whisk in the mustard and season with salt and freshly ground black pepper.
Peel Spanish onions and cut into thick slices. Separate slices into rings.
Dip each ring in batter and shallow-fry in hot vegetable oil for 1-2 mins, turning once, until golden brown.
Drain well and serve with a dip.
Notes
Can quite easily polish the lot off in one go!!!! An old relative who used to hate the European version of onion rings called them Yankee so they stood out.