Heat the olive oil and butter in a large pot over medium heat, and saute the leeks, cauliflower, and garlic for about 10 minutes. Stir in the vegetable broth, and bring the mixture to a boil. Reduce heat, cover, and simmer 45 minutes.
Remove the soup from heat. Blend the soup with an immersion blender or hand mixer. Season with salt and pepper. Mix in the heavy cream, and continue blending until smooth.