Preheat a grill to medium-high.
Meanwhile, cut the Romaine into large wedges. If the head is very large, quarter it. If it’s a bit smaller, use two and portion it into halves. Gently rub the lettuce with the grapeseed oil, then sprinkle over the celery seed and cumin and season generously with salt and pepper.
Use tongs to place the lettuce on the preheated grill and cook on each side, turning over once lightly charred and wilted—about 45 seconds per side. Transfer the Romaine to a serving plate, drizzle with herb vinaigrette and serve with a fork and knife.