This dairy soup would be great for Shavuot.
I found several variations of Polish Pickle Soup, requested by Benita Lee. The simplest one, from an old cookbook of mine, Polish Cookery by Marja Ochorowicz-Monatowa (Crown Publishers, 1958), calls for 3-4 dill pickles (depending on size), peeled and thinly sliced, to be cooked in 2 tablespoons butter and 1 cup light meat stock (substitute vegetable stock to make it kosher) until transparent and completely tender (about 30 minutes). Thoroughly blend 1 tablespoon flour with ¾ cup sour cream; add to pickle mixture, along with 5 more cups of stock. (The recipe does not say anything about heating the soup further, but I would say it should be simmered for a few minutes to combine everything and heat it through. Don't let it boil after adding the sour cream or it will curdle.) Season sparingly-the author advises that salt accentuates the sour taste, but notes that you should add a little pickling liquid or a few drops of lemon juice if the soup is not tart enough. Serves 6 to 7.
Most of the other recipes I found call for potatoes; additional ingredients in some recipes include carrots, dill, even dried mushrooms (boletus).
The following recipe is a template, combining several I found; it includes lots of suggestions for variations to adjust according to your taste. I am hoping one of these options proves pretty close to the one in Benita's taste memory.
1. In a large pot, slowly sauté 1 cup chopped onion in 2 tablespoons butter until translucent and tender. If you'd like, you can also add ½ to 1 cup each carrots (grated or diced) and celery (diced) and 1-3 teaspoons finely minced garlic (no need for garlic if the pickles you'll be using are garlic dills).
2. Add 4 cups vegetable broth [can use chicken broth, if kosher is not a concern], 3 cups dill pickles (choose regular or garlic dills; you can either chop them or shred in a food processor or on a box grater), about ½ cup pickle liquid from the jar, and 2 cups potatoes, peeled and diced. [Another option is to start the recipe here, either omitting the onions and other vegetables from step 1, or simply adding them without sautéing them first in butter.) Bring everything to a boil, reduce the heat to medium-low, and cook until the potatoes are tender.
3. In a medium bowl, thoroughly blend 2 tablespoons all-purpose flour or Wondra with 1 cup sour cream. OR beat 1 egg with 1 cup sour cream until thoroughly blended. (These mixtures are used to enrich the soup and thicken it slightly.) Slowly stir some of the hot soup into the sour cream mixture (this tempering will prevent curdling). Then whisk the tempered sour cream mixture into the rest of the soup. Simmer slowly for a few minutes, stirring, until the soup is heated through and slightly thickened. Taste, and adjust seasoning, adding salt, pepper, and up to ½ cup more of the pickle liquid until you reach your ideal level of tartness.
4. You can serve the soup chunky as is, garnished, if you wish, with fresh dill and a dollop of sour cream. (You can also add some dill to the soup near the end of the cooking time in Step 3). Or you can puree the soup until smooth in a food processor or blender, then garnish as you prefer.
Serves about 6