Blanch the haricots verts in a large pot of boiling salted water for 1.5 minutes only. Using a spider, a metal strainer or tongs Remove the haricots verts and immerse directly into a bowl of ice water. Drain and set aside.
In the same boiling water cook the corn and eggs for 4.5 - 5 minutes only. Remove and immerse in a bowl of ice water. Drain and set aside.
For the vinaigrette, combine the vinegar, mustard, salt, and pepper. Slowly whisk in the olive oil to make an emulsion. Mix in the chopped dill and set aside.
Add the potatoes to the pot of boiling water. Lower the heat to a rolling simmer and cook for 8-10 minutes, until the potatoes are just tender when pierced with a knife. Drain the potatoes in a colander, then place the colander with the potatoes over the empty pot off the heat and cover with the lid. Leave the potatoes for 5 minutes, to dry out. Then gently toss in a bowl with salt, pepper and 2 tablespoons of the dressing. Set aside.
Season the skin of the fish generously with salt, and the flesh with salt & pepper.
Preheat a skillet over medium heat for 3-4 minutes. Add 2 tablespoons of olive oil to the pan. When the oil begins to shimmer, lay the fish down in the pan. Cook it for 5 minutes on the skin side, then flip it over to cook for 30 seconds on the other side. Remove salmon and set aside.
Arrange the lettuce, arugula, salmon, potatoes, haricots verts, tomatoes, corn, cucumbers, and eggs on a large flat platter. Drizzle some vinaigrette over the fish and vegetables and serve the rest on the side with the olives, capers, and marinated onions (if using) and dill sprigs.