Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer) . Wash potatoes thoroughly and cut in half. Mix with salt, pepper and 2 tbsp. oil on a baking tray, distribute evenly. Bake in a hot oven for about 40 minutes. After about 10 minutes, spread bacon on top.
Meanwhile cook the beans in boiling salted water for about 8 minutes. Drain, quench and let drain. Wash and dice the tomatoes.
Mix vinegar with salt, pepper and sugar well. Fold in 4 tablespoons of oil. Take potatoes out of the oven, remove bacon. Mix potatoes, beans, tomatoes and vinaigrette in a bowl. Let the salad steep for about 15 minutes.
Drain the tuna and pluck it, carefully fold it into the salad. Season salad to taste. Break the bacon roughly into pieces and spread on top.