Whisk together champagne vinegar, Dijon mustard, salt and pepper.
Slowly drizzle in olive oil until combined, until thickened and emulsified.
Set aside.
Boil a saucepan of water, add green beans, and cook for about 3 or until bright green.
Drain, and place in an ice bath, drain again and set aside.
Refill saucepan with cold water, and add potatoes.
Bring to a boil, and then reduce to a simmer.
Cook until potatoes are tender, about 15-20.
Drain the potatoes and let them cool.
Toss the potatoes with just enough dressing to coat.
Place eggs in a small saucepan of cold water.
Bring to a boil and cover.
Remove from heat.
Set aside for 8.
Place in cold water to cool, then peel and slice in half.
Assemble the salad in a large dish.
Arrange lettuce, green beans, eggs, tomatoes, olives, and tuna.
Drizzle with dressing and sprinkle with parsley right before serving.