Heat the 2 tablespoons of unsalted butter in a large, heavy-bottomed soup pot or Dutch oven over medium heat until melted and foaming.

Add the finely diced white onion and cook, stirring occasionally, until deeply soft and completely translucent, about 5 minutes.

Pour in the ½ cup of dry sherry to aggressively deglaze the pot, scraping up any caramelized sugars from the onions. Cook vigorously until the sharp alcohol scent burns off and almost all of the liquid disappears into a thick syrup, about 4 minutes.

Immediately stir the thawed sweet peas and fresh baby spinach directly into the hot onion and sherry reduction. Cook just until the spinach wilts slightly.

Stir in the 2 tablespoons of fresh lemon juice and drop the whole fresh dill sprigs into the pot.

Pour in 3 cups of the chicken broth. Bring the liquid to a rolling boil over medium-high heat.

Once boiling, reduce the heat to low, cover partially, and simmer the soup gently for 20 minutes. This critical thermal phase ensures the tough outer skins of the peas break down entirely, softening the vegetables for a flawless puree.

Remove the pot from the heat and allow the soup to cool slightly to room temperature to prevent thermal expansion blowouts.

Using a high-speed immersion blender directly in the pot (or working carefully in batches with a standard blender), aggressively puree the softened vegetables and broth until they transform into a completely smooth, velvety, bright green liquid.

Return the pureed soup to the pot over low heat. If the emulsion is too thick, stream in the remaining 1 cup of chicken broth to achieve your desired consistency.

Vigorously stir in the kosher salt, coarse black pepper, and the ½ cup of heavy whipping cream. Do not allow the soup to boil once the cream is added, or the dairy may break and separate.

Ladle the hot soup into shallow bowls and garnish heavily with a dollop of cold sour cream and a tiny sprig of fresh dill to provide a sharp thermal and visual contrast.