Preheat the oven. Arrange a rack in the middle of the oven. Heat the oven to 415ºF/212ºC.
Roast the cauliflower and the aromatics. Sprinkle the cauliflower, quartered onion and garlic cloves with salt, drizzle with olive oil, and arrange them in a single layer on a baking sheet. Make sure you have a large enough baking sheet (use two sheets if necessary), so the vegetables aren’t too crowded and don’t steam instead of roasting. Roast the cauliflower and aromatics until caramelized and golden at the edges, 15-20 minutes. The bake time for roasted cauliflower will depend on the size and thickness of the cauliflower. Larger florets can take up to 25 minutes to turn golden while small florets can be done in as little as 15 minutes.
Blend. Add the roasted cauliflower, roasted aromatics, cashews, vegetable broth, lemon juice, coriander, and salt into a high-speed blender and blend on high until completely smooth.
Season. Taste and adjust the flavor as needed, adding more salt for saltiness, lemon juice for acidity, and coriander for a citrusy-nutty flavor.
Serve. Pour the soup into serving bowls - it will be hot straight out of the blender (if using Vitamix). Top with a few roasted cauliflower florets, chopped chives or green onions (optional), and a sprinkle of black pepper.
Store. Leftover soup keeps well in the refrigerator for 3-4 days. For longer term storage, freeze in an airtight container for up to 3 months.
Reheat. Transfer frozen soup into the refrigerator 24 hours before reheating to thaw slightly. Reheat in a pot on the stovetop over medium-low heat, stirring frequently, until warmed through. Add 1/4 cup/60 ml water at a time if too thick.