I was really pleased with the balance of flavors in this soup. It's a good showcase for celeriac's nutty, celery-y flavor, and the rosemary and bacon round it out and make it hearty enough for a light main course. I served this with Whole Wheat Popovers, which went really well with it.
2 Tbsp butter
1 large onion, chopped
5-6 cloves garlic, minced
3 lbs celeriac, peeled and cubed
1 1/2 Tbsp chopped fresh rosemary
Up to 1 cup milk
4-6 oz cooked bacon, chopped
1 Tbsp white wine vinegar
Salt to taste
Melt the butter in a soup pot, then add the onion and garlic and saute over medium-high heat for several minutes, until slightly browned. Add the celeriac and rosemary, then enough stock to just cover the vegetables. Bring to a boil, then reduce heat and simmer until the celeriac is very tender, about 10-15 minutes. (If you have not cooked the bacon ahead of time, this is a good time to do it. I like to do large-ish quantities under the broiler; I line a rimmed pan with foil then place a cooking rack over it and the bacon on the rack. Broil for 3-4 minutes per side.)
Once the celeriac is tender, puree the soup. I do this with an immersion blender, but you could also do it in batches in a regular blender. Once pureed, stir in milk to reach the desired consistency, then add the bacon and white wine vinegar. Season with salt to taste. Serve hot.