Serves 4 as a main course.
Separate garlic cloves, leaving skins intact. In a large kettle cover potatoes with salted water by 2 inches and add garlic. Simmer potatoes and garlic until potatoes are just tender, about 20 minutes. Drain potatoes and garlic in a colander and, when cool enough to handle, cut potatoes into 1/4-inch-thick slices. Peel garlic and in a blender puree with water, vinegar, oil, mustard, and salt and pepper to taste. Potatoes may be prepared and dressing made 1 day ahead and chilled separately, covered.
Cut beans into 1-inch pieces. Have ready a bowl of ice and cold water. In a large saucepan of boiling salted water cook beans until crisp-tender, about 3 minutes for haricots verts or 6 minutes for regular green beans. In a colander drain beans and plunge into ice water to stop cooking. Pat beans dry.Cut bell pepper into 1/4-inch dice and halve cucumber lengthwise. Seed cucumber and diagonally cut into 1/4-inch-thick slices. Pit and chop olives. Vegetables and olives may be prepared up to this point 4 hours ahead and chilled separately, covered.
Drain tuna and transfer with potatoes, beans, bell pepper, cucumber, and olives to a large bowl.
Just before serving, toss salad with dressing and crumble feta on top.