Arrange a rack in the middle of the oven and heat to 425℉. Line a rimmed baking sheet with parchment paper; set aside. Coarsely grate a third of the cauliflower florets into rice-size pieces and set aside in a bowl.
Break the remaining cauliflower into florets about 1 ¼ inches wide, and place in a large bowl. Add the onion and any cauliflower leaves. Drizzle with 2 tablespoons of the oil, sprinkle with ¼ teaspoon of the salt, and toss to combine. Transfer to the baking sheet and spread into an even layer.
Roast until cooked through and golden brown, about 20 minutes. Remove from the oven and set aside to cool.
Transfer the roasted vegetables to a large bowl. Add the reserved grated cauliflower, parsley, mint, tarragon, pomegranate seeds, pistachios, lemon juice, cumin, remaining 3 tablespoons oil, and remaining ¼ teaspoon salt. Toss gently just to combine. Transfer to a platter and serve.
