1. Preheat oven to 400 degrees F. Coat celery root with olive oil and roast on a pan for 20-30 minutes, shaking once or twice. Heat olive oil in a large soup pot over medium heat.
2. Add leeks to hot oil and sauté until soft and translucent, 5-8 minutes. Stir often. Add garlic and cook for 1 minute more. Add potatoes to the leek and garlic mixture and add broth to cover by 1".
3. Raise heat to high and bring broth to a boil. Cover and cook potato mixture until fork soft, 8-10 minutes. Add roasted celery root to potatoes and boil until soft, 3-5 minutes.
4. Remove pot from heat and use a potato masher to mash mixture. Add additional broth, if needed, and milk. Use an immersion blender to blend until soup is creamy and smooth. Add additional broth or milk to reach your desired thickness. Stir in salt and pepper, to taste.
Source of recipe: I came up with this easy recipe to use up some winter farmer's market veggies.