Combine spices in a small bowl. In a medium saucepan, heat butter and oil over medium heat. Add leeks and onion; cook until translucent, about 8 minutes. Sprinkle with spices; add sweet potatoes, squash, and carrots. Stir well to combine.
Add chicken stock to the mixture in the saucepan, and simmer over low heat, partially covered, until vegetables are tender, about 40 minutes. Season with salt and pepper. Remove from heat.
Transfer half the soup to a blender, working in batches, if necessary, so as not to fill jar more than halfway; process until smooth. Return puree to saucepan; place over low heat until heated through. Serve hot; garnish each bowl with a handful of radish sprouts, if using.