How to Make It
In a large bowl, mix the mayonnaise with the lightly beaten egg, scallion greens, capers, Dijon mustard, 2 tablespoons of the lemon juice and 1/2 teaspoon of pepper. Add the smoked trout and toss to combine. Fold in 1/2 cup of the panko.
Spread the remaining 1/2 cup of panko on a plate. Form the trout mixture into eight 1-inch-thick cakes, add them to the plate and turn to coat with the panko.
In a large skillet, heat 2 tablespoons of the olive oil until shimmering. Add the cakes to the skillet and cook over moderate heat, turning once, until browned and heated through, about 6 minutes. Transfer the trout cakes to plates.
In a medium bowl, whisk the remaining 1 1/2 tablespoons of olive oil with the remaining 1 tablespoon of lemon juice and season with salt and pepper. Add the mâche and toss to coat. Transfer the salad to the plates and serve with lemon wedges.