Season the sliced shallots with salt and pepper, add the sugar and the vinegar and leave to marinate for a few minutes.
In a large mixing bowl, combine the tarragon and grapes and grate in most of the cheese. Drain the pickled onions and throw them in with a little of the vinegary juices, salt and pepper and a glug of olive oil. Toss everything together and divide between four plates. Grate the rest of the cheese on top of each salad and serve.