1) Position a rack 15cm from the grill and preheat.
2) Thinly slice the fennel lengthwise, using a mandolin if you have one. Soak the cut fennel in a bowl of very cold water.
3) Smash the garlic cloves, sprinkle with the one teaspoon of 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.
4) Place the lamb, trimmed-side down, on the grilling rack. Grill the meat until nicely browned, about five minutes. Turn and cook until an instant-read thermometer inserted into the sides of the meat registers 50°C, another two to three minutes. Set the meat aside to rest while preparing the salad.
5) Pit and cut the plums and peach into chunks with their skins still on. Toss the fruit in a salad bowl with the mesclun salad greens. Drain and pat the fennel dry, add to the salad.
6) Thinly slice the lamb against the grain into bite-sized pieces and add that to the salad as well. Season the salad with the salt and pepper, toss with the dressing and divide evenly among four plates.
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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