Preheat oven to 425°F (220°C).

In a large bowl, mix 2 Tbsp olive oil, 1/2 tsp salt, and 2 Tbsp harissa. Add cauliflower and toss well to coat.

Spread cauliflower on a rimmed baking sheet and roast for 35 minutes, tossing halfway, until tender and golden.

Remove from oven, toss with remaining 2 Tbsp harissa, and return to oven for 5–7 minutes until caramelized.

In a small bowl, whisk remaining 2 Tbsp olive oil, tahini, honey, 1 Tbsp lemon juice, remaining salt, and black pepper. Whisk in 1 Tbsp water to thin.

Transfer cauliflower to a serving dish, drizzle with tahini-honey sauce, and finish with remaining lemon juice. Garnish with parsley if desired.