In a large pot, heat butter, olive oil, and garlic over medium heat until fragrant. Add leeks and cauliflower, and saute for about 10 minutes.
Add chicken broth and bring to a boil. Reduce heat, cover, and simmer for 30 minutes or until vegetables are soft.
Blend soup with an immersion blender. Stir in heavy cream until smooth. Season with salt and pepper to taste.
Notes
To thicken soup make a slurry with 1 tablespoon cornstarch mixed with 1 tablespoon cold water, and add in Step 3.Use a potato masher for a more chunky style soup.Refrigerate leftovers covered for up to 4 days, or freeze cooled portions in zippered bags for up to 6 weeks. Serve leftovers chilled or reheat using low heat on the stovetop.