500g/1lb 2oz cooked beetroot, thinly sliced
2 tbsp chervil sprigs (or parsley)
1 tbsp lemon juice
2 banana shallots, finely sliced into rings
50g/1戮oz baby watercress sprigs
1 tbsp French mustard
1 tsp caster sugar
1 tbsp olive oil
3 tbsp olive oil
pinch salt
2 tbsp rice wine vinegar
4 mackerel, filleted