In a medium pot, heat the oil over moderate heat. Add the leek, onion, celery, and garlic to the pot, and season with the salt and pepper. Sauté for 5 minutes, stirring occasionally, or until vegetables become translucent and softened.

Add the cauliflower and broth, turn the heat to high and bring to a boil. Once boiling, reduce the heat to the lowest setting, cover the pot, and cook the soup for 10 to 15 minutes, or until the cauliflower is very soft and fork tender. Remove the pot from the heat and stir in the frozen peas and baby spinach, allowing the spinach to wilt and the peas to defrost from the residual heat.

Using an immersion blender, traditional blender or food processor*, puree the soup until smooth, or until it reaches the consistency you desire. Transfer the pureed soup to a large pitcher or bowl to chill in the fridge. (Alternatively, this soup is also delicious warm). Ladle into bowls and serve with a dollop of optional Greek yogurt and a sprinkle of julienned fresh basil.