2 eating apples, quartered and finely sliced
3 large sticks of celery, plus celery leaves
250g/9oz seedless red grapes, halved
around 2 tsp fresh lemon juice
4 tsp lemon juice
2 tbsp flatleaf parsley, finely chopped
2 tsp Dijon mustard
freshly ground black pepper
flaked sea salt and freshly ground black pepper
½ tsp caster sugar
4 tsp olive oil
¼ tsp flaked sea salt
150ml/5fl oz sunflower oil
65g/2¼oz shelled walnut halves
1 tbsp white wine vinegar
100ml/3½fl oz crème fraîche
2 large free-range egg yolks
8 fresh mackerel fillets