Prepare the cauliflower and asparagus: chop bottoms off the asparagus stalks and discard, slice the remaining stalks into 2 inch long spears. Chop the cauliflower into florets. Spread cauliflower and asparagus onto a large baking sheet and drizzle with 1 tbsp olive oil. Salt and pepper. Place inside the oven to roast for approximately 25 mins, gently turning the vegetables halfway through.
Meanwhile, heat up a large soup pot to medium heat. Add 2 tbsp olive oil, chopped onion, sliced mushrooms, and garlic. Sauté vegetables for 8-10 minutes, or until flagrant.
Add asparagus and cauliflower to soup pot. Cover with broth and bring to a boil. Reduce heat to low and simmer for about 10 minutes.
Add lemon juice, zest, and basil leaves. Using an immersion blender, puree until smooth. Alternatively, cool down the soup a bit, and pour into a large blender or food processor to puree.
Serve immediately and garnish with any of the following optional toppings.
Optional toppings: Parmesan cheese, lemon pepper, pesto, or even pancetta.