Thinly Sliced Scallions And Crushed Potato Chips (For Serving)
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 the 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 lose their structure, 10–15 minutes.
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 of soup.)
Season with more salt and pepper and warm over medium-low until heated through.
Serve soup topped with scallions and potato chips.