How to Make It
Place extra-virgin olive oil, grapeseed oil, garlic, coriander seeds, and turmeric in a blender. Process on high speed until smooth, about 35 seconds.
Place chicken, breast side down, on a cutting board. Using poultry shears, cut along both sides of backbone; discard backbone. Turn chicken breast side up, and press down on breastbone to flatten chicken. Season both sides with salt. Place chicken in a large roasting pan, and rub all over with marinade. Cover and chill at least 2 hours and up to 8 hours or overnight.
Preheat a gas grill to medium-high (about 450°F) on one side, or push hot coals to one side of a charcoal grill. Gently blot chicken with paper towels to remove excess marinade. Place chicken, breast side up, on oiled grates over unlit side of grill. Grill, covered, over indirect heat until chicken is well browned and an instant-read thermometer inserted in thickest portion of chicken registers 160°F, about 20 minutes per side. Transfer chicken to a carving board, and let rest 15 minutes before carving.