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, and 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.