How to Make It
Put the potatoes and carrot in a steamer basket set over a pot of simmering water and steam until just tender, about 9 minutes. Sprinkle the frozen peas on top and steam just until heated through, 1 minute longer. Transfer the vegetables to a plate and refrigerate until they are cooled slightly, about 10 minutes.
Meanwhile, in a medium bowl, whisk the mayonnaise with the crème fraîche, mustard and 1 tablespoon of the lemon juice; season with salt and pepper. Fold in the chopped parsley, chives, flaked tuna and cooled vegetables.
In a medium bowl, whisk the olive oil with the remaining 1 tablespoon of lemon juice and season with salt and pepper. Add the arugula and toss. Mound the greens on plates, top with the tuna and potato salad and serve.