Combine Greek yogurt, harissa, garlic, and lime zest; season with ½ teaspoon salt and ¼ teaspoon pepper.
Trim the leaves from 1 medium cauliflower. Rub the cauliflower with olive oil. Roast at 400°F until tender, 1 hour.
Sprinkle with chopped cilantro. Slice or break into pieces and serve with the harissa yogurt.