12 spears asparagus, trimmed
½ onion, finely chopped
50g/2oz flatleaf parsley, leaves only
1 white turnip, thinly sliced on a mandoline
150g/5oz watercress leaves
2 tbsp baby watercress leaves
3 tbsp extra virgin olive oil
1 tsp salt
1 tsp sugar
250ml/9fl oz vegetable stock
110ml/4fl oz white wine vinegar
50g/2oz butter
4 mackerel fillets