Preheat oven to 450°F.
Place chopped 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.
Spread the sweet potatoes and shallots out on a foil or lined baking sheet. Roast for 25 minutes or until cooked through and nicely caramelized and browned around the edges. Remove from oven.
Meanwhile, heat chicken stock in a saucepan and heat.
Place the cooked sweet potatoes and shallots in a blender and add the hot chicken stock. Purée until smooth, working in batches.
Taste for seasoning, add more salt and pepper to taste. Serve with yogurt or sour cream if desired.