Preparation:

Cook potato with skin on in a saucepan covered with cold water. (Don’t heat the water too fast or the skin will break. The whole process should take about 20 minutes.) In another saucepan, place eggs in cold water and bring to a simmer, then cook for 10 minutes.

While potato and eggs are cooking, prepare bell pepper by removing stem and core/seeds. Slice one half of pepper into thin strips. Cut each tomato into six wedges and reserve.

Wash and drain lettuce and reserve. Then drain tuna and remove from can – try to keep tuna intact. 
When potato is done, remove from saucepan with slotted spoon and let cool down some. Use the same water to quickly blanch the string beans. Once cooked, run beans under cold water in a colander to cool.

Prepare vinaigrette by drizzling olive oil into vinegar while constantly whisking. When just warm to the touch, peel the potato. Slice the potato and immediately dress with vinaigrette. Peel hard-boiled eggs and cut each in half length-wise. 

Assemble salad by dressing lettuce and placing on a platter in layers. Place tuna in center of bed of lettuce and arrange all other components around tuna. Drizzle remaining dressing over tuna and other vegetables. Season with salt and pepper.