Preheat oven to 450°F.
Prepare the sweet potatoes: Place sweet potato cubes and shallots in a large bowl. Drizzle with oil. Sprinkle with salt, thyme, and cumin. Toss so all pieces are well coated with olive oil and seasonings.
Roast the sweet potatoes: Spread the sweet potatoes and shallots out on a foil or silicone-lined baking sheet. Roast for 25 minutes or until cooked through and nicely caramelized and browned around the edges. Remove from oven.
Heat the chicken stock: While the sweet potatoes are roasting, heat chicken stock in a saucepan on the stovetop until steamy.
Blend the sweet potatoes: Place the cooked sweet potatoes and shallots in a blender and add the hot chicken stock. Purée until smooth. NOTE: when blending hot liquids, fill the blender bowl no more than a third full, and start with short pulses. So, depending on the size of your blender you may need to work in batches. Or use an immersion blender.
Serve: Taste for seasoning, add more salt and pepper to taste. Serve in bowls with a dollop of sour cream (if using). Leftovers will keep for about a week, or freeze for up to three months.
