Cut the potatoes into quarters (or smaller if you prefer), boil in salted water for 20 minutes (or less if you cut it smaller). Put the haricot vert in the pot with the potatoes for the last couple of minutes to cook them.
Fill a pot of water and add the eggs, turn it to high heat. When the water comes to a boil turn the heat down to medium/medium high and set a timer for 6.5 minutes. When the time is up take the eggs out and put them in a bowl of cold water to stop the cooking process.
Wash and cut the lettuce into smaller bits. Cut the tomato into 12 wedges
Open and drain the cans of tuna
Add mustard, vinegar, oil, salt and pepper in a jar. Shake for 30 seconds until well combined.
Put the lettuce in a bowl and toss with a little bit of the vinaigrette. Place the lettuce on a plate or in a bowl and put the rest of the ingredients onto it. Pour the rest of the vinaigrette over the salad. Serve immediately.