This reminds me of happy days in the South of France eating lunch on some harbour, looking at the boats from under the shade of an umbrella, drawing on the paper tablecloth and slowly becoming drowsy with little carafes of chilled white wine. Mackerel is an excellent replacement for tuna.
Preheat the grill to full blast.
First make the dressing. Strip the leaves from the rosemary (you should be left with roughly a teaspoonful) and put in a blender with the egg yolks, garlic, mustard, vinegar and lemon juice. Blend until well combined, then slowly, in a thin dribble, add the oil. If the dressing appears too thick, add a splash of hot water and blend until the dressing reaches a double cream-like pouring consistency. Leave to one side while the other ingredients are prepared.
Place the mackerel, seasoned and lightly oiled, on a foil-lined grill tray. Grill for 4â€“5 minutes on each side until cooked. Allow to cool.
Cook the potatoes in a large saucepan of simmering water for about 15 minutes until tender, adding the beans for the last 5 minutes of cooking time. Drain in a colander, then immediately plunge into a large bowl of iced water and leave to cool completely.
Lower the eggs gently into another saucepan of boiling water. Return to the boil and cook for 8 minutes. Cool quickly under running water until cold. Peel the eggs and cut in half. They should be only just hard-boiled with the yolk moist and a rich golden-yellow colour. Remove and discard the mackerel skin and take the flesh from the bones in nice large flakes. Be extra careful when extracting it, as it would be nice to eat the salad without regularly sticking your fingers in your mouth to remove the bones under cover of your napkin. Chuck the bones. Drain the potatoes and cut into fairly thick slices. Cut the anchovies in half lengthways and the tomatoes through the middle. Roughly tear the lettuce leaves. Toss the mackerel pieces, potatoes, green beans, lettuce, anchovies, tomatoes, olives, capers and basil leaves loosely together in a large salad bowl. Put the halved eggs here and there before liberally spooning over the dressing. Eat with a cold bottle of rosÃ©.