To make the dressing whisk the vinegar with the mustard, salt and pepper and gradually whisk in the oil. Taste and adjust seasonings, then stir in the shallot and set aside until needed.
As soon as the beetroot is cooked and cut up, toss with enough of the dressing to coat nicely then leave to cool. Just before serving add the parsley leaves and more dressing if you think it could do with it and then arrange on a shallow serving dish. Scatter over walnuts and feta and serve.
sounds good to me