Bring a small saucepan of water to the boil, and then carefully place the eggs into the water. Allow them to boil for 8 minutes, remove the pot from the heat, and place the eggs in a bowl of cold water.
While the eggs cook, bring a saucepan filled with water to a boil. Add the potatoes, and cook for 20 minutes, until they are soft, and a knife can easily pass through them. Drain the saucepan, and run the potatoes under cold water to cool them. Slice any large potatoes in half.
While the potatoes boil, place the green beans in a steamer, and steam them for 4 minutes, until they are just starting to soften. Place them in cold water to cool, and stop the cooking process.
In a pestle and mortar, crush the garlic, along with the flaked sea salt. Once a paste has formed, mix in the Dijon mustard, and then mix in the lemon juice and the black pepper. Slowly whisk in the olive oil, and then stir in the chopped chives and dill.
Gently roll or tap the cooled boiled eggs on a hard surface to crack the shells, and then peel and halve them, discarding the egg shell.
On a large platter or in a large bowl, arrange the cooled potatoes, the cooled green beans, the boiled eggs, tinned tuna, heirloom tomatoes, red onion, black olives, and capers. Finish off by drizzling the Niçoise salad with the lemon herb vinaigrette.