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1
Place the lamb in a large ceramic baking dish or roasting pan.
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2
Combine the buttermilk, mustard, olive oil, garlic, rosemary, lemon juice, and salt and pepper in a small bowl.
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3
Rub the mixture all over the lamb, cover with plastic wrap, and place in the fridge.
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4
Let marinate for 12 hours or overnight.
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5
Remove the lamb from the fridge to allow it to come up to room temperature.
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6
Rub the grill grates with an oil-coated paper towel.
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7
Heat your grill to medium-high, using charcoal or gas.
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8
Place the lamb on the grill away from the flame (using indirect heat).
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9
Grill, covered, turning periodically, for 1 hour 15 minutes, or until a thermometer inserted into the thickest part of the lamb (without touching bone) reads 135 degrees F.
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10
Remove the lamb from the grill, and loosely cover with foil.
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11
Allow to rest for 20 minutes before carving.
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12
Carve by holding the lamb up vertically by the bone and slicing down, across the grain, into thin slices.
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13
Spread slices on a platter, and drizzle with the mint vinaigrette.
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14
Whisk together the shallot, mint, vinegar, and lemon juice in a small bowl.
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15
Whisk in the olive oil, and season with red-pepper flakes, salt, and pepper.
