How to Make It
In a blender, combine the lemon juice with the vinegar, shallot, mustard, garlic and lemon zest and pulse until finely chopped. With the machine on, add the olive oil in a thin stream and blend until emulsified. Add the capers and a pinch each of salt and pepper and pulse until the capers are coarsely chopped.
Put the potatoes in a large pot of cold water. Bring to a boil and cook until tender, about 18 minutes. Using a slotted spoon, transfer the potatoes to a colander and rinse under cold water until cool. Pat dry and cut into quarters.
Return the water to a boil. Add the haricots verts and cook until crisp-tender, about 5 minutes. Drain and refresh under cold water. Pat dry.
In a large bowl, toss the salad greens with 1/4 cup of the lemon-caper dressing and transfer to large plates. Working with 1 ingredient at a time, toss the potatoes, haricots verts, trout and tomatoes with the remaining dressing and arrange over the salad greens. Garnish with the eggs and olives. Serve the salad immediately, passing any remaining dressing separately.