For the salad:
For the dressing:
1. Preparations: peel the potatoes, cut into chunks, place in a large pot, cover with water, season with 1 teaspoon of salt, bring to a boil and cook until fork tender. Drain and let cool. Boil the eggs for 10 minutes, peel and set aside. Cook the green beans until ready. Set aside.
2. Thinly slice the onion, cut in half cherry tomatoes and hard boiled eggs. Set aside.
3. Place all vegetables in a large bowl and set aside.
4. Make the dressing: in a medium size bowl whisk mustard, vinegar, salt and pepper. While whisking slowly pour olive oil. Set aside the dressing.
5. Mix all the vegetables with half of the dressing.
6. Lay the lettuce leaves onto a large plate. Transfer the salad to the plate.
7. Season the tuna with salt and pepper from both sides. Heat olive oil over medium heat and cook the tuna 2 minutes from each side. Slice and place on top of the salad.
8. Cut boiled eggs in half and add to the salad. Season with salt and pepper. Drizzle the remaining dressing.