Coarsely chopping the onion and apples and finely chopping the celeriac and potatoes will ensure that the vegetables all cook at about the same rate. Or, put the celeriac and potatoes in five or so minutes before the onion and apple.

2 – 3 T olive oil
1 medium celeriac, peeled and finely chopped
2 apples, (peeled) cored and coarsely chopped
1 large onion, peeled and coarsely chopped
1 large or 2 medium-small (red potatoes), finely chopped
6 c broth, plus up to 2 more cups broth (or water) for thinning, optional
1 t celery seed, optional
salt and pepper, to taste
1 or 2 T lime or lemon juice, optional
fresh, chopped scallion tops, chives, or parsley, for garnish

1. In a large soup pot, heat olive oil. Add celeriac, apples, onion, and potatoes. Sauté over medium-low heat until vegetables are becoming tender, about 10 minutes. Add the 6 cups broth or water, celery seed, if using, salt, and pepper; raise heat and bring to a boil. Turn heat down and simmer for about 40 minutes, until all the vegetables are very tender.

2. Purée, in batches, in a food processor. You’ll have to put the puréed soup in a bowl until the soup pot is empty. Pour puréed soup back into pot. Or, use an immersion blender to  purée the soup in the soup pot. Add up to 2 cups more broth or water until the soup has the consistency you want. Heat over medium-low heat until hot. Add a tablespoon of lime or lemon juice. Taste, and add another tablespoon if you like. Taste for seasonings as well. Serve in soup bowl garnished with chopped scallion tops or herbs.