Step 1

Heat oil in a large saucepan over medium-low. Add onions, celery, and garlic and cook, stirring often and adding a splash or so of water if the bottom of pan gets too brown, until golden brown, 30–40 minutes. Season with salt and pepper. Add cashews, sugar, red pepper flakes, and 3 cups water and bring mixture to a simmer. Cook until vegetables are very soft and losing their structure, 10–15 minutes.

Step 2

Blend one-third of sugar snap peas with one-third of vegetable mixture in a blender, adding some cooking liquid from vegetable mixture as needed to thin, until very smooth, about 2 minutes. Press purée through a fine-mesh sieve into a medium saucepan; discard solids. Working in 2 batches, repeat with remaining peas and vegetable mixture, adding water as needed if you run out of cooking liquid. If soup is still very thick, thin with water until you get a velvety, pourable consistency. (You should have about 7 cups soup.)

Step 3

Stir vinegar into soup; season with more salt and pepper and warm over medium-low until heated through.

Step 4

Serve soup topped with scallions and potato chips.

Step 5

Do Ahead: Soup can be made 2 days ahead. Let cool; cover and chill. Reheat over medium-low, thinning with water as needed until pourable.