For the onion rings, place the vegetable oil into a deep, heavy-bottomed saucepan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place both types of flour into a large bowl, season well with salt and freshly ground black pepper and whisk in enough sparkling water to make a batter the consistency of single cream - you may not need it all.
Dip the onion rings into the batter then carefully place them into the hot oil to deep fry for 4-5 minutes, or until golden-brown. Remove the onion rings from the oil with a slotted spoon and drain on kitchen paper.
For the salad, place the mustards, vinegar and olive oil into a large bowl, season well with salt and freshly ground black pepper and mix well. Add the watercress to the bowl and stir to coat.
To serve, place the salad onto a large plate with the onion rings on top.