Step 1

Mix 2 lb. russet or Yukon Gold potatoes (about 4 large), scrubbed, thinly sliced, 1 lb. white onions (about 2 medium), thinly sliced, 6 garlic cloves, coarsely chopped, ¼ cup extra-virgin olive oil, 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, 1 tsp. dried thyme, and 1 cup water in a large Dutch oven or other heavy pot to combine. Cover and cook over medium heat, stirring occasionally and adjusting heat as need to keep vegetables from browning, until onions are very tender and translucent, 12–18 minutes.

Step 2

Add 4 cups low-sodium chicken broth to pot, re-cover, and cook until potatoes are tender and falling apart, 12–18 minutes. Remove from heat and stir in ½ cup sour cream.

Step 3

Using an immersion blender, blend soup in pot until mostly smooth; you want some small lumps and pieces of skin. (Alternatively, you can purée soup, in batches in a food processor, transferring to a medium bowl as you go. Return soup to pot before proceeding.)

Step 4

Stir 2 tsp. distilled white vinegar and ½ tsp. freshly ground black pepper into soup and reheat over medium, stirring often, until hot but not bubbling (boiling might curdle the soup). Taste and season with more salt and/or pepper if desired.

Step 5

Ladle soup among bowls. Top each with a dollop of sour cream and scatter scallions, pickled jalapeños, and dill over. Drizzle with oil.Do ahead: Soup (without toppings) can be made 3 days ahead. Let cool; cover and chill. Reheat soup over medium-low, adding a splash or so of water to thin as needed. Season with more salt and pepper if desired.