Position a rack 6 inches from the broiler and preheat. Thinly slice the fennel lengthwise, using a mandoline if you have one. Soak the cut fennel in a bowl of very cold water. Smash the garlic cloves, sprinkle with the 1 teaspoon salt, and, with the flat side of a large knife, mash and smear the mixture to a coarse paste. Stir the garlic, olive oil, and marjoram together and rub all over the lamb. Season with pepper, to taste. Place the lamb, trimmed-side down on a broiler pan. Broil the meat until nicely browned, about 5 minutes. Turn and cook until an instant-read thermometer inserted into the sides of the meat registers 120 degrees F, another 2 to 3 minutes. Set meat aside to rest while preparing the salad.
Pit and cut the plums and peach into chunks with their skins. Toss fruit in a salad bowl with the mesclun salad greens. Drain and pat the fennel dry, add to the salad.
Thinly slice the lamb against the grain into bite-size pieces, and add that to the salad as well. Season salad with the salt and pepper, toss with the dressing, and divide evenly among 4 plates. Serve.
Whisk the vinegar, mustard, salt, and pepper, to taste, in a small bowl. Gradually whisk in the oil, starting with a few drops and then adding the rest in a steady stream, to make a smooth, slightly thick dressing.
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