For the dressing:
1. Preheat the oven to 200 degrees Celsius and line a baking tray with baking paper.
2. Cut the potatoes in half, coat them with olive oil and roast them in the oven until golden and crispy. This will take about 25-30 minutes, depending on size.
3. In the meantime, bring a small pot of water to boil and blanch the green beans in it for 30 seconds. Remove them from the pot and place them in an ice bath to stop them from cooking further.
4. Using the same pot, boil the eggs for 7-9 minutes, then place them in an ice bath and let them cool for a few minutes. Peel them and cut them in half.
5. Roughly chop up the lettuce then toss it in a bowl along with olives, capers, tuna, cherry tomatoes, basil leaves and green beans.
6. When the potatoes are ready, add them to the bowl too.
7. Whisk lemon juice, olive oil, and mustard together and pour over the salad. Season with salt and pepper and enjoy immediately.