Heat the butter in a stockpot over medium heat until melted. Add the onions, carrots, and squash and sauté for 5 minutes, until the onions start to soften. Season with salt and pepper. Add the broth and bring to a simmer. Simmer partially covered for 25 to 30 minutes, until the vegetables are all very soft.
While the soup is coming to a simmer, make the Indian Spice Blend. In a small bowl, combine the garlic, ginger, coriander, cumin, cinnamon, cayenne, turmeric, cloves, salt, and pepper. Add it to the simmering vegetables in broth for the last 10 to 15 minutes.
Working in batches, puree the soup in a blender or food processor (alternately, you can run the mixture through a food mill, which offers a great kind of nubby texture, or puree with an immersion blender). Return the pureed soup to the pot.
Add the cream and heat just until warmed through. Garnish with chopped scallions if desired. Serve warm.
Notes
If you use vegetable broth, the soup is vegetarian, and if there is a dairy intolerance in your household, swap the butter out for olive oil and skip the cream — you’ll have a lighter, lactose-free, vegan soup. If you like fresh cilantro, think about sprinkling some of the chopped herb on top just before serving.
This is a great make-ahead soup. It can be stored for up to 4 days. If you plan to make it ahead, stop before the cream is added, refrigerate the soup, then heat and add the cream right before serving. If you reheat it with the cream already added in, make sure you only bring it to a gentle simmer over medium-low heat so it doesn’t curdle.