How to Make It
Make the sauce In a small bowl, stir the crème fraîche with the dill and lemon juice. Season with salt and pepper. Refrigerate until ready to serve.
Make the dressing In a medium saucepan, cook the butter over moderate heat, swirling the pan occasionally, until browned, about 4 minutes. Add the capers and fry until crisp, about 1 minute. Using a slotted spoon, transfer to a paper towel–lined plate and reserve. Strain the browned butter into a small bowl. Whisk in the vinegar and mustard, then slowly whisk in the olive oil until emulsified. Season with salt and pepper. Keep warm.
Make the salad Fill a medium bowl with ice water. In a medium saucepan of salted boiling water, cook the haricots verts until crisp-tender, about 4 minutes. Drain and transfer to the ice water. Drain and pat dry; halve the beans crosswise. In a large bowl, combine the haricots verts and reserved capers with both lettuces, the arugula, watercress, radishes, scallions, dill and tomatoes. Gently toss with the dressing. Spread the dill sauce on plates. Mound the salad and salmon on top; serve.