Preheat your oven to 400 degrees
Prepare the cauliflower and asparagus: chop off bottoms of 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 15-25 mins, gently turning the vegetables halfway through...the asparagus will be done before the cauliflower so take it out first to cool
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 the 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