In a large pot or Dutch oven (at least 6 quarts) set over medium-high heat, warm the olive oil until shimmering.

Add the onions and cook, stirring, until they begin to soften, about 5 minutes.

Add the cashews and cook, stirring, until the onions are translucent and the cashews have slightly browned, about 3 minutes.

Stir in the garlic and ginger, and cook for 30 seconds.

Add the squash, broth, ginger, cumin, coriander, curry powder, turmeric and red pepper flakes, and stir to combine.

Season to taste with salt and pepper, and bring the soup to a simmer.

Reduce the heat to low, cover the pot, and cook the soup until the squash is easily pierced with a knife, 20 to 25 minutes.

Using an immersion blender, purée soup in pot until smooth. Alternatively, you can transfer the soup to a blender and purée starting on low speed and increasing it to high. You may need to do this in batches. Transfer soup from the blender back to the pot.

Stir in coconut milk and cook over low heat, covered, until slightly thickened, for about 15 to 20 minutes.

Serve immediately or refrigerate until ready. If serving the soup later, while reheating the soup, you may need to thin the soup with more broth or coconut milk to reach the desired consistency.