Heat a large stockpot over medium-high heat. Add onion, leeks, sweet potatoes, garlic and butter; cook, stirring occasionally, until vegetables are fork-tender and have started to brown, about 10-12 minutes.

Add white wine and deglaze the pan, scraping up the brown bits from the bottom of the pan. Add broth, bay leaf and paprika and stir to combine. Reduce heat to low and simmer 25 minutes, stirring occasionally.

Using a ladle, remove half the vegetables from the pan and reserve. Using an immersion hand blender, blend the remaining vegetables and broth in the pot into a puree. Return vegetables to pot and add chickpeas and the leaves from a few sprigs of oregano; stir to combine. Ladle soup into serving bowls.

In a small saucepan over medium heat, add cashews and 1 tablespoon butter. Fry cashews in butter until warm and just lightly browned on the edges, about 3-4 minutes. Remove from heat and sprinkle cashews over soup in bowls. Top each bowl with a dollop of sour cream or creme fraiche; garnish with more fresh oregano, if desired. Serve immediately.