Add potatoes, onions, and broth to the slow cooker. Sprinkle it with salt and pepper and stir. Cook on high for 3-4 hours or until potatoes are tender.
About 30 minutes before soup will be done, pour boiling water over cashews JUST until they are covered and let rest for 10 minutes to soften.
In a high speed blender, puree cashews AND water until completely blended. Set aside.
In a medium saucepan, heat olive oil over medium high heat.
Add in flour, whisking continuously until bubbly.
Add in pureed cashews and sour cream, whisking until smooth.
Add to soup and stir to mix.
Cook for 10 more minutes
Remove half of the soup to a bowl and, using an immersion blender, puree the remaining soup until smooth (alternatively, add half of soup to blender, cover lid with a towel and blend until smooth)
Combine blended and unblended soup, stirring to combine.
Taste, season with salt and pepper if desired.
Serve with sliced chives and scallions (or bacon and cheddar if not vegan).
Add broth to loosen during reheating.