Bring a large pot of water to a boil over high heat. Add broccoli and cauliflower; cook for 3 minutes. Remove 2 cups of broccoli and cauliflower florets with a slotted spoon; set aside. Add spinach to the pot; cook until the broccoli and cauliflower are tender and the spinach has wilted for about 2 minutes. Drain. Working in batches if needed, transfer the mixture to a blender; add broth and shallots. Secure the lid on the blender and remove the centerpiece to allow steam to escape. Place a clean towel over the opening. Process until smooth, about 30 seconds. (Use caution when blending hot liquids. Alternatively, transfer the drained broccoli and cauliflower back to the pot; add broth and shallots and process using an immersion blender.)
Thank you to Eating Well for this great recipe!