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1
Mince garlic, then mash to a paste with salt. Stir together garlic paste, oil, lemon zest and juice, thyme, pepper, and allspice. Put lamb, fat side up, in a large ceramic or glass dish. Using a paring knife, make 1 1/2-inch-long slits (about 2 inches deep) 2 inches apart all over leg. Reserve 1 teaspoon thyme mixture and push remainder into slits, then rub all over with reserved teaspoon of mixture. Marinate lamb, covered and chilled, 12 hours.
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2
Let lamb stand at room temperature 30 minutes before grilling.
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3
Prepare grill for indirect-heat cooking over medium-hot charcoal (high heat for gas). If using a gas grill, preheat all burners on high, covered, 10 minutes, then turn off all burners except burner closest to you.
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4
Oil grill rack, then grill lamb over area with no coals (or over turned-off burner or burners), covered, without turning, until thermometer inserted into thickest part of roast (almost to the bone but not touching it) registers 125u00b0F for medium-rare, 1 1/4 to 1 1/2 hours (temperatures in thinner parts of leg may register up to 160u00b0F). To maintain medium-hot charcoal, add more charcoal (about 2 cups) every 15 minutes.
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5
Let lamb stand on a cutting board, uncovered, 30 minutes.
