Boil new potatoes till fork-tender, about 10 minutes. Drain and let cool.
Place the pan back on the heat and fill halfway with water. Bring to the boil, then add the green beans. Boil for 3 to 5 minutes, until tender but crisp. Drain immediately and immerse in a bath of ice water.
Pan-fry tuna in frying pan with a bit of olive oil. Season well with salt and pepper. I like to cook my tuna all the way through, but cook it as you like it.
Start assembling the salad: Lay lettuce leaves over a large serving plate. Scatter the tomatoes, green beans and potatoes around the edges. Place the tuna steaks and anchovies in the centre.
Make the dressing by whisking together the olive oil, balsamic vinegar, salt, pepper, sugar and English mustard. Pour over salad, then serve!