Bring a large saucepan of water to the boil. Cook potatoes for 12-15 minutes or until tender. Drain and transfer to a large bowl.
Heat one tablespoon oil in a large frying pan over high heat. Add green beans and cook for 3-4 minutes or until lightly charred. Add garlic and half the anchovies and cook for a further minute or until garlic is golden. Transfer to bowl with potatoes.
Combine 2 tablespoons oil, red wine vinegar, Dijon mustard and remaining anchovies in a small bowl and whisk until emulsified. Pour dressing over warm potatoes and beans and toss well to coat.
Wipe out pan and return to medium heat. Add tuna and cook 30 seconds each side or until golden on the outside but very rare inside. Slice thinly and set aside.
To serve, lay out tomatoes, cos wedges, onion, olives, capers and chives on a large serving platter. Top with potatoes, beans and tuna slices.