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1
At least 1 day ahead, marinate the lamb: Set the lamb flat on a large cutting board.
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2
Trim any excess fat and open like a book to make an even thickness all around.
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3
Lay a piece of plastic wrap on top of the lamb; pound to flatten slightly to make a more even thickness.
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4
Cut the meat into 2 or 3 more manageable pieces.
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5
Sprinkle all over with 1 T. of the salt and 1 t. of the pepper.
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6
In a small bowl, mix the mustard, garlic, thyme, rosemary and the remaining 1 T. of the salt and 1 t. pepper.
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7
Spread all over the lamb, transfer to a large nonreactive dish, cover and refrigerate for at least 24 hours and up to 2 days.
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8
Just before grilling, make the sauce: In a medium bowl, whisk 1/4 cup water with the sugar, salt and pepper - they don't have to dissolve completely.
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9
Stir in the mint, vinegar and oil.
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10
Let sit while the lamb grills.
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11
Taste and season with more sugar, salt and pepper if needed (the sauce should have a sharp, acidic tang to complement the rich lamb).
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12
Grill the lamb: Heat a gas grill to medium high or prepare a charcoal fire with hot and medium-hot areas.
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13
Put the lamb on the grill (on the hotter part if using a charcoal fire) and cook (covered if on a gas grill) without disturbing it until it's nicely browned, about 6-8 minutes.
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14
Flip, reduce the heat on the gas grill to medium (or move to the cooler part of the charcoal fire), and cook until and instant-read thermometer inserted into a thicker part of the lamb registers 130 degrees F for med-rare, 5-8 more minutes.
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15
Tranfer the lamb to a cutting board, tent with foil, and let rest for 10 minutes.
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16
Slice thinly across the grain and serve with the mint sauce.
