Preheat the oven to 450F.

Line a large baking sheet with foil and spray with oil. Place cauliflower and halved Brussels sprouts on the baking sheet, drizzle with olive oil, and roast for 25 minutes, tossing halfway until slightly browned.

Meanwhile, melt the butter in a large sauce pan over low heat, add chopped shallots, and cook until translucent about 5 minutes. Add salt and vegetable broth, then simmer for 5 minutes.

Remove the roasted vegetables from the oven, reserve about 1 cup for topping, and keep warm. Transfer the rest to the pot and simmer for 2 minutes.

Transfer the soup in batches to blender and blend until smooth. Serve in bowls topped with reserved roasted vegetables and black pepper.