On rimmed baking tray in preheated oven, place pecans and pumpkin seeds and toast, stirring once or twice, until pecans are fragrant and pumpkin seeds are toasted, 6 to 8 minutes. Let cool for 5 minutes.
Increase oven temperature to 425 F. Transfer cooled nuts to plate and place empty tray back in oven to preheat for 10 minutes.
Trim cauliflower and cut into florets. Transfer florets to large bowl along with 3 Tbsp oil, cumin, salt, and pepper and toss until well combined. Set aside.
Take garlic head and trim about 1/2 inch off the top, exposing cloves within. Place on piece of parchment paper large enough to wrap garlic completely. Drizzle exposed garlic cloves with remaining 1 Tbsp oil. Bring parchment paper up and twist top to secure. Cover parchment-wrapped garlic bulb with foil in the same manner.
Tip seasoned cauliflower onto warmed baking tray and spread into even layer. Place wrapped garlic bulb on tray with cauliflower before roasting everything together, stirring once halfway, until cauliflower is caramelized and garlic cloves are soft, about 40 minutes.
While cauliflower is roasting, make crumble. On cutting board, place toasted pecans and pumpkin seeds with Medjool dates and roughly chop. Add arugula or kale, hemp hearts, and lemon zest and continue to chop until arugula is in bite-sized pieces and mixture is well combined but still has plenty of texture.
Transfer warm roasted cauliflower to serving platter. Squeeze garlic overtop cauliflower, tossing, if desired. Drizzle with lemon juice before sprinkling with crumble. Enjoy while warm or at room temperature.
Per serving: 477 calories; 11 g protein; 30 g total fat (3 g sat. fat); 50 g total carbohydrates (32 g sugars, 11 g fiber); 359 mg sodium