My CSA share didn't provide us with cauliflower this year or the last (although though they do grow it at the farm) and I had already planned to make butternut squash ravioli and was making a kabocha squash cheesecake, so I didn't want to overdo it on the squash. What else could I use for an autumnal soup? The big celery root staring me in the face answered that question.
Last year I turned the celery root that showed up in my CSA share into a raw salad along with fennel - a constant in my share in the fall - and apples, which are pretty much the only fruit that we get for the last few months of our deliveries. I really do like that salad because the flavors complement each other so well - the earthiness of the celery root, the tartness of the apple, and the mellow herbaceousness of the fennel - and because it's really crisp and refreshing. But I didn't want to make it a second year in a row and I already had a raw root vegetable dish in mind (I refer you to this photo, top row, 2nd from right).
So I took all those elements from the salad and turned them into a soup. Makes sense, right? Well, I reserved that judgment until I had pureed and did some final seasoning, but, thankfully, I was right. The soup had an earthy richness to it from the celery root but was also light courtesy of the fennel and apple, and a little fresh lemon juice at the end brightened it all up. Another nice thing about this soup? It's super easy to make - everything gets chopped while the other components are cooking, and you don't have to be particularly precise about it because it eventually all gets blended.
I think any bowl of soup on its own is kind of boring, so I topped this one with fennel fronds (root to tip cooking?) and tiny little cubes of fresh apple for a bit of textural contrast and acidity. I like something crunchy with soup - be it chopped toasted nuts or seeds or a cracker - so I made blue cheese and walnut tuiles to go along with this. The tuiles are a bit of a pain to make; a good alternative would be to crumble a little bit of blue cheese and crushed toasted walnuts onto the soup before serving, which is honestly what I would do next time because I really hate making tuiles (it may have something to do with this).
Celery Root, Fennel, and Apple Soup
makes approximately 8 cups
vegan if using olive oil; gluten-free
1 small white onion
2 tablespoons (1 ounce) unsalted butter (use olive oil to make it vegan)
½ pound fennel bulb (from approx 12 oz stalk)
1 pound celery root
2 cups vegetable stock
2 cups water
1 pound tart apples (about 3 medium apples)
1 teaspoon lemon juice, divided
Chop the onion; you should have about ½ cup.
Melt the butter in a large pot set over medium heat. Add the onions and a bit of salt and cook until the onions are soft, about 5-8 minutes. If the onions start to brown, lower the heat.
Cut the stalks and root end off the fennel bulb and reserve the fronds. Thinly slice the bulb, then add it to the pot along with a bit more salt and cook until the fennel has started to soften, another 5-8 minutes.
Peel and chop the celery root into ½” chunks (they don’t have to be precise, but the smaller the pieces are, the faster the soup will cook). Once the fennel has softened, add the vegetable stock, water, celery root, some more salt, and a few grinds of black pepper. Bring the mixture to a boil, then cover the pot and simmer until the celery root is easily pierced with a knife, about 15 minutes.
When the celery root is almost done cooking, core and peel the apples. Reserve ¼ of one apple and chop the remaining apples. Add the chopped apples to the pot and continue cooking for another 5 minutes. Finely dice the reserved apple and toss with the lemon juice. [Note: if you are storing the soup to serve at a later time, chop all of the apples and add them to the pot; dice a piece of a fresh apple before serving. The chopped fennel fronds will keep for a few days in the refrigerator; wrap them in a napkin and place inside a resealable bag or container.]
While the soup is cooking, pull the fennel fronds off the stalks and finely chop the fronds. Puree the soup in a blender or using an immersion blender. To make the soup completely smooth, strain it through a fine mesh sieve. (This is optional, but it makes the soup much smoother and only takes a few extra minutes.) The soup can be refrigerated for 3 days or frozen for up to 2 weeks. Before serving, stir in ½ teaspoon of lemon juice and add more salt to taste.
To serve, top each bowl with 1 teaspoon of the diced apples and a sprinkle of fennel fronds.