2 heads of garlic, top third cut off
6 Tbsp olive oil, divided
1 3 – 4 lb chicken, cut in half lengthwise, backbone removed
1/4 Cup Za’atar
1-1/2 tsp lemon zest
3 Tbsp lemon juice (preferably fresh)
1 Tbsp Rosemary leaf (preferably fresh)
1 small Serrano Chile, seeded and minced
2 tsp Marjoram leaf
Kosher salt and freshly ground Black Peppercorns
Prepare the marinade: Preheat oven to 400°F. Line a baking sheet with aluminum foil, and place the two heads of garlic (with their tops sliced off) on the sheet. Drizzle carefully with 1 Tbsp of the olive oil, and gather up the edges of the foil to wrap the garlic cloves tightly. Roast about 45 min, until garlic heads are tender and golden brown. Remove baking sheet from oven, open foil packet, and allow garlic to cool.
Once garlic has cooled (at least 30 min), place chicken in a large rectangular (13″ x 9″) glass baking dish, and sprinkle with 2-1/2 Tbsp za’atar.
Squeeze contents of roasted garlic cloves out of their skins and into a small bowl and, using the back of a fork (or using a mortar and pestle) mash garlic into a paste. Whisk in 4 Tbsp olive oil, the lemon zest and juice, rosemary, serrano chile, and marjoram. Pour garlic-lemon-spice mixture over the chicken and turn chicken over a few times to thoroughly coat with marinade. Cover chicken with plastic wrap and refrigerate overnight.
Grill the chicken: Prepare grill and heat to medium-high heat. Unwrap the marinated chicken and season lightly with kosher salt and black pepper; let chicken come to room temperature, at least 30 min. Lightly oil grill rack with remaining olive oil before placing chicken on the grill. Grill chicken for 30-40 min, turning occasionally. Chicken is done when skin is crispy and brown, and an instant-read thermometer (carefully inserted into the deepest part of thigh without touching the bone) registers 160°.
Remove chicken from grill to a large cutting board and allow to “rest” for 10 min. Garnish chicken with remaining za’atar before carving each chicken half into 4 pieces. Serve immediately.