Add the eggs to a small saucepan and cover with cold water. Bring the pan to the boil, then reduce to a simmer for 7 minutes.
Whilst you are simmering the eggs you can cook the green beans. Trim the tough ends from the beans and add to a pan of boiling salted water. Boil for 3-4 minutes until tender. Drain and set aside.
Drain the hot water from the eggs and rinse immediately under cold water. Once cool enough to handle, peel the eggs, slice in half and set aside.
Add the cooked and cooled beans to a large bowl with the baby spinach.
Quarter the tomatoes, halve the olives, removing any stones and finely slice the scallions. Add to the mixing bowl with the spinach and green beans.
Flake the tuna into the mixing bowl and chop the anchovies into small bite sized chunks. Toss the salad to combine all the ingredients.
Transfer the salad to a large serving dish or individual plates and top with the sliced eggs.
To make the dressing, finely chop the capers and add to a small mixing bowl. Add the lemon juice, olive oil, mustard and mayonnaise. Stir well to combine.
Drizzle the dressing over the salad to serve.