Mix the oil, vinegar, mustard and seasoning together.
To assemble the salad, place the potatoes, green beans, tuna, tomatoes and leaves in a large bowl, mix with half the dressing, divide between four plates.
Cut the eggs in four and add to each plate. Divide the black olives between each plate.
Mix the remaining dressing with the mayonnaise and the basil. Drizzle the dressing over each plate and top with the anchovy strips. Very good with crusty bread.
The Niçoise Salad could also be served from one large salad bowl.