Cook the green beans in a pan of boiling water for 4 minutes. Plunge them into a bowl of very cold water.
Cook the eggs in a pan of boiling water for 8 minutes. Drain in a sieve under cold running water until cold. Peel and cut the eggs into quarters.
For the dressing, whisk the oil, vinegar and mustard in a large bowl until thick. Season with black pepper.
Add the green beans, mixed beans, cherry tomatoes and spring onions to the dressing and mix. Flake the tuna on top and add the hard-boiled eggs. Season with black pepper.
Use red kidney beans if you can’t find mixed beans in your local store.