Step 1

Whisk together oil, salt, turmeric, nigella seeds, and coriander in a large bowl. Add cauliflower; gently toss, turning and brushing as needed, until thoroughly coated. Let marinated cauliflower rest at room temperature 1 hour.

Step 2

Meanwhile, open bottom vent of a grill completely. Fill a charcoal chimney starter with briquettes, and place on one side of bottom grate of grill. Place log directly next to chimney starter; light charcoal chimney starter filled with briquettes. When briquettes are covered with gray ash and part of the log closest to chimney starter is smoldering, pour briquettes onto the side of the bottom grate where the chimney starter was sitting. Lightly grease top grate with grapeseed oil; place on grill.

Step 3

Place cauliflower on oiled grate over side without coals, reserving any marinade. Grill, covered, until cauliflower is tender and a wooden skewer slides into center of cauliflower with just a little resistance, about 1 hour to 1 hour and 15 minutes, brushing with any reserved marinade and rotating 180 degrees halfway through cook time. Transfer cauliflower to grate directly over coals; grill, uncovered, turning occasionally, until golden brown and lightly charred, 5 to 10 minutes.

Step 4

Place cauliflower on a serving platter. Drizzle each cauliflower head with 1/2 cup tahina and 1/2 cup zhough, and top each head with 1/2 cup sumac onions. Sprinkle cauliflower with sea salt and tender fresh herbs. Serve remaining tahina, zough, and sumac onions with cauliflower.