Place the new potatoes in a saucepan and cover with cold water. Bring to the boil and simmer for 15 minutes or until tender. Add the beans for the final 2 minutes, then remove and run under cold water until cooled. Drain the potatoes and leave to cool.
Place the eggs in a saucepan and cover with water. Bring to the boil, turn down the heat and simmer for 7 minutes, then drain and run under cold water to cool.
Halve the cucumber and scrape out the seeds. Cut into 1cm pieces and place in a large bowl with the cherry tomatoes, olives, spring onions, red pepper and lettuce leaves.
To make the dressing, place the oil, vinegar, mustard and garlic clove in a small jug or bowl and whisk together. Season to taste.
Heat a griddle pan over a medium/high heat. Brush the tuna steaks with the vegetable oil and season. Place the steaks on the griddle pan and cook for 3 minutes on each side or until there are dark golden griddle marks and it the fish has turned opaque on the bottom half. Carefully turn them over and repeat on the other side until just cooked through. Set aside to cool slightly, then slice.
Peel and halve the hard-boiled eggs. Slice the cooled potatoes into quarters and add to the salad leaves. Pour over the dressing and combine.
Divide the dressed salad between 4 plates. Top with the sliced tuna and the halved eggs, then crumble over the feta.