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
Roast the cauliflower and leeks with a drizzle of balsamic vinegar until caramelized and tender before using. This will add a depth of flavor to the soup.
To thicken soup:
blend leftover mashed potatoes into the soup, in step 3.
make a slurry with 1 tablespoon corn starch 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.