Preheat the oven to 500 degrees F.
To make the marinade for the salmon, mix the lemon juice, lemon zest, olive oil, mustard, garlic, 1 1/2 tablespoons salt, and 1/2 tablespoon pepper in a small bowl and set aside
Cover the bottom of the baking dish with aluminum foil then place the salmon in it.
Grease the marinade over the salmon and make sure to cover all the topping of the salmon.
Leave the salmon aside for about 15 minutes to absorb all the marinade flavor.
In these moments, place the potatoes and 2 tablespoons of salt in a large pot of water.
Bring the water to a boil, then lower the heat and simmer for 10 to 15 minutes, until the potatoes are barely tender when pierced with a knife.
Now, drain the potatoes in a colander, then place the colander with the potatoes over the empty pot off the heat and cover with a clean and dry kitchen towel.
Leave the potatoes to steam for about 15 to 20 minutes, until tender buts still firm.
When the potatoes are cool enough to handle, slice them in thick slices (about 1/2 inch) and set them aside.
Place the salmon in the oven and cook for 12 to 15 minutes at 500°F, or until it is almost cooked through.
Once the salmon is cooked, remove it from the oven and place it on a plate and allow it to rest for 15 minutes.
For serving you need to remove the skin and break it into large pieces.
Blanch the haricot verts in a large pot of boiling salted water for about 2 minutes only.
Drain immediately and immerse in a bowl of ice water to stop the cooking, then drain them again and set aside.
For making the vinaigrette, mix the vinegar, mustard, salt, and pepper, then slowly whisk in the olive oil to make an emulsion.
To serve the salad Nicoise, arrange the salmon in the middle, potatoes, haricot verts, tomatoes, eggs, watercress, olives, and anchovies, I recommend you to use a large flat platter.
Pour some vinaigrette over the fish and vegetables and serve the rest in a pitcher on the side.