In a large soup pot, sauté the onions with a few tablespoons of water and a big pinch of salt for 7 to 10 minutes, stirring often and adding water, a couple of tablespoons at a time, to prevent burning if needed.
Now add the garlic, pumpkin, sage leaves, thyme, and red pepper flakes. Stir and sauté for another minute or two.
Now add the vegetable broth and bring to a boil. Once boiling, reduce to a simmer and simmer partially covered for 15 minutes or until the pumpkin is very tender. Remove the soup from heat.
Drain the cashews, discard the soaking liquid, and add them to a high-speed blender. Transfer the soup to the blender too and blend until completely smooth (do this in batches if needed). Pour the soup back into the pot and season to taste. If adding chickpeas or spinach, stir in now.
Divide among bowls and garnish with thinned sour cream (see notes) and/or red pepper flakes.