Preheat oven to 425°F. Spread cauliflower on a large baking sheet and toss with 2 Tbsp. olive oil, 1 Tbsp. harissa, and ¼ tsp. salt. Bake for 20 minutes.
Remove cauliflower from the oven, toss, and brush with remaining 2 Tbsp. harissa paste. Push cauliflower to one half of the baking sheet, and add chickpeas and shallots to remaining half.Toss chickpeas and shallots with remaining 1 Tbsp. olive oil, ¼ tsp. salt, and black pepper. Place baking sheet back in the oven for another 15 to 20 minutes, until crisp.
Meanwhile, prepare Lemon-Tahini Dressing by combining all ingredients in a small bowl and stirring to combine. Gradually stream in 1 to 2 Tbsp. of warm water, whisking constantly, until dressing is smooth and creamy.
Assemble salad by transferring roasted cauliflower, chickpeas, and shallots to a large bowl or serving platter. Toss with fresh herbs, and drizzle with Lemon-Tahini Dressing.
