Heat the oil in a medium pot over medium heat. Add the spring onion and cook for 1 to 2 minutes, or until soft. Add the potatoes, garlic, salt, pepper, and water and simmer for 12 minutes or until the potatoes are fork tender.
Add the asparagus to the pot of potatoes and continue simmering for 5 more minutes. Remove the pot from the heat and let cool for 5 minutes. Stir in the peas and then transfer the soup to a blender. Add the mustard, lemon juice, and lemon zest and blend until creamy. Add the basil and blend until combined. Season to taste.
If desired you can make mint & pine nut topping by chopping together pine nuts, mint, red pepper flakes, and salt.
Portion the soup into bowls and sprinkle with the mint & pine nut topping, drizzles of olive oil, a few sprigs of fresh mint and serve with crusty bread.
