1 boneless leg of lamb, butterflied to even 2-inch thickness and trimmed of excess fat, about 6 pounds total
8 medium garlic cloves, peeled, divided
1/2 cup whole grain Dijon mustard
1/2 cup extra-virgin olive oil
1/4 cup dry white wine
2 tablespoons finely chopped fresh rosemary
2 tablespoons fresh juice from 1 lemon
Thinly slice 4 cloves of garlic. Spread lamb open on cutting board. Using tip of a paring knife, make 1/2-inch-deep slits all over lamb. Insert garlic slices into slits in lamb.
Combine remaining 4 cloves of garlic, mustard, olive oil, white wine, rosemary, and lemon juice in the workbowl of a food processor. Pulse until coarse puree forms.
Place lamb in a large shallow container and spread half of mustard mixture over top. Flip lamb and spread with remaining mustard mixture. Cover and let sit in refrigerator overnight.
Remove lamb from refrigerator 1 to 2 hours before grilling, allowing to come to room temperature. Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and spread the coals evenly over entire surface of coal grate. Set cooking grate in place, cover gill and allow to preheat for 5 minutes. Clean and oil the grilling grate. Grill lamb over high heat until well seared, 8 to 10 minutes. Flip and continue to cook until second side is seared and meat registers 130 degrees on an instant read thermometer inserted into thickest part of lamb, 8 to 10 minutes more. Transfer to a cutting board and let rest for 15 minutes. Slice and serve immediately.